Heroes & Villains MBTI Glossary


Here’s a quick guide to the terms and jargon used on this blog and other MBTI forums, discussions, and writing. These are just straight-ahead, unembellished definitions. For more in-depth descriptions, see the links and read the character posts.

Cognitive Functions

Character traits first observed and named by Carl Jung, and classified into eight functions. Isabel Briggs-Meyer and Katherine Cook-Briggs then arranged the functions into 16 groups, or types. Functions are the building blocks of MBTI, and each type has four. MBTI nerds study the functions to type themselves rather than the letters of their type’s name.

To learn the eight cognitive functions themselves, start here: The Cognitive Functions (with Clue suspects).


The unique arrangement of a type’s four functions. Each stack starts with the dominant function, and works its way down to the inferior. Learning your stack helps you understand your own wiring and how you can best grow into your type.

Dominant Function

The first function in your type’s stack. This is the trait you are most easily defined by, the function you find most natural and powerful. It’s your home base. Abbreviated as -dom, as in “Fi-dom” or “Se-dom” to indicate either the function itself or the person who leads with that function.

Auxiliary Function

The second function in your type’s stack. This trait shows up early in life and supports and expands upon the use of your dominant function. Most stereotypical descriptions of the types work off the interplay of the first two functions in the stack. Abbreviated as -aux, as in “Te-aux” or “Si-aux” to indicate either the function itself or the person who uses it in the second slot of their stack.

Tertiary Function

The third function in your type’s stack. Always the opposite of the auxiliary function, it provides more perspective and more options for your type to work from. Usually shows up a little later in life, perhaps post-adolescence, and can cause disruptions and inner conflict as the person adjusts to its unfamiliar influence. (Can be abbreviated -tert, but not used as much as -dom or -aux.)

Inferior Function

The fourth and lowest function in your type’s stack. Always the opposite of your dominant function. It challenges the way you normally do things, and can cause stress and confusion as the person confronts this unknown side of themselves. Sometimes referred to as the Aspirational Function, as its effective use signals true growth in the user. (Can be abbreviated -inf, but not used as much as -dom or-aux.)



The person of a particular type being discussed, usually in the context of how their functions manifest. Often found in phrases like, “The Fi-user is very private.”


A cognitive function used to make decisions. Includes both Thinking and Feeling, and their Introverted and Extraverted versions.


A cognitive function used to collect or understand information. Includes both Intuition and Sensing, and their Introverted and Extraverted versions.


A cognitive function of the Judging kind that favors logic or rationality in its decision-making. Not the same as being intellectual, or even smart. Can be Introverted (Ti) or Extraverted (Te).


A cognitive function of the Judging kind that favors values or emotional well-being in its decision-making. Not the same as being emotional, or even moral. Can be Introverted (Fi) or Extraverted (Fe).


A cognitive function of the Perceiving kind, tuned to physical stimulus, activity, or experience as it understands the world. Not the same as a psychic-like perception or “Spidey-Sense,” but rather based on physical sensation and the five senses. Can be Introverted (Si) or Extraverted (Se).


A cognitive function of the Perceiving kind, tuned to abstract connections or possibilities as it understands the world. Not the same as gut instinct or psychic-like perception, or even creativity, but rather based on a conceptual interpretation of the world. Can be Introverted (Ni) or Extraverted (Ne).


A cognitive function focused on the user’s personal perspective or inner framework. An Introvert is a person whose dominant function is Introverted.

Extraverted (or “Extroverted,” in the modern spelling)

A cognitive function focused on the objective experience and standards outside of the user. An Extravert is a person whose dominant function is Extraverted.



The effective use of a person’s functions, from the top to the bottom of their stack. A mature, well-rounded person of any type looks less and less like their stereotype, and will appear more balanced. They will operate from their dominant function first, but they can employ both Thinking and Feeling, both Sensing and Intuition, and are comfortable accessing both Introverted and Extraverted functions.


Any number of mal-adapted uses of the functions. Not necessarily a synonym for mental or emotional illness. The person may simply not be in touch with their lower functions, or may be using their higher functions too aggressively. They may also be in grip or loop behavior.


Under extreme stress, when their typical strategies have failed, a person may fall into the unhealthy use of their inferior function. This is called “being in the grip.” They may be either uncharacteristically domineering or withdrawn, depending on their usual behavior. They often become like a stereotyped version of their opposite type.


Another unhealthy behavior, a loop finds a person cycling between their first and third functions (dominant and tertiary). Since these two functions always share the same direction (Introverted or Extraverted), this will put the user in a loop, unable to access the perspective of the complementary functions.

An Introverted loop usually finds the user withdrawn and unable to connect with the outside world. An Extraverted loop usually finds the user acting impulsively and aggressively, unable to consult their inner values or reason. Loops, according to MBTI rules, only happen with the first and third functions, typically because the user is more accustomed to using the direction of their first function, and falls out of touch with their auxiliary.



An axis is a pair of functions that is always found together. If you have one, you have the other. If you are Fi dominant, you will have Te in the inferior, and vice-versa. Even types with completely different letters may share the same axis, just reversed. When examined, they may actually have similar ways of viewing the world, just opposite priorities.

The axes prevent two of one kind of function from showing up in a single type. You can’t have both Ti and Te, because every type needs a Thinking and a Feeling function in order to operate sanely in the world. So a Ti-user will have to have Fe somewhere in their stack. Likewise, you can’t have both Ni and Si, because you need an Extraverted function to collect information from the outside world, lest you have nothing but your own inner assumptions to work from. So Ni is paired with Se, and Si is paired with Ne.

Axes are often represented like this:

Fi-Te/Te-Fi (depending on which of the two functions is higher in the user’s stack)




xSTJ, etc.

If an MBTI writer/blogger hasn’t quite nailed down the type of the character they’re discussing, or they want to refer to a collection of types that share similar functions and letters, they may insert an “X” in the name of the type. This is not the same as saying someone is between types, because strict MBTI theory states that you don’t change type.


Shadow Functions

This is the sub-theory that states that we all use all eight of the functions, regardless of type. Basically, the other four functions not in your type exist in a “shadow” version of your stack, below your normal stack, and manifest in times of extreme stress or growth opportunities. Some versions of this theory break the stack into pairs of functions, with each pair working like one function in the standard theory.

It’s needlessly complicated to me. I believe, along with other MBTI nerds, that the functions we have are more than adequate, and they can often pair up to produce “faux” versions of the other functions, producing the illusion of shadow functions. But that’s much more than this blog has time, or desire, to cover.

Ambivert, or INFP/J, etc.

Terms used by MBTI newbies who find themselves waffling being different types based on conflicting results from free online tests. Most tests work off of strict dichotomies between the four letters in your type’s name, rather than the cognitive functions. So a test may tell someone they are 80% E, when in fact they often feel Introverted. A healthy understanding of the functions will reveal that we all have two Extraverted and two Introverted functions, and we move back and forth between them every day. Likewise, many types with only one letter difference actually have no cognitive functions in common, so you can’t be an INFJ one day and an INFP the next, strictly speaking.


The preference most online tests, blogs, and forums give to certain types and functions. Most descriptions make the Intuitive types sound creative and intelligent, while the Sensors are literal-minded and boring. Introverts are special, and Extraverts are shallow. Thinkers are geniuses, and Feelers are manipulative basket cases.

And so on.

This is the major reason that everyone reports as an INTJ, even though it’s supposed to be one of the rarest types (even I did it, and I’m an INFP). Recognizing not only the usefulness, but the power and scope of every function and type will make typing yourself and others more interesting and rewarding. Every type has both Extraverted and Introverted functions, both Thinking and Feeling, both Sensing and Intuition, strengths and weaknesses, and so no type should have anything to be ashamed or jealous of.