What is a good word for the identifying characteristics of something?

Question

For example, let us say we were conducting a study of facial features and we were comparing hair color, nose type and eye shape. Those three characteristics would identify something or someone. What would a good term be for the items that are used to identify a unique point in the problem space?

I looked at a few similar words in the Thesaurus on thefreedictionary.com. For example, under coordinate (n) it yielded co-ordinate, Cartesian coordinate, polar coordinate and number. Most of those were not even synonyms except in the most trivial sense.

"Characteristics" is a superset of what I am referring to, as it includes all characteristics, not simply the ones considered unique or defining.

Answer 1

When I worked in scientific research, we called these distinction sets (for a group) or comparators (for the individual parameters).

You might also call them distinguishing features or points of differentiation/distinction.

Answer 2

The classic term in our “problem space” is probably Roman Jakobson’s distinctive feature, one of a small set of acoustic or articulatory features which collectively distinguish the phonemes within any language’s phonological system.

The concept has been usefully extended to other disciplines — Lévi-Strauss, for instance, employed it fundamentally in the structural study of kinship systems and mythological corpora. But linguists may be vexed if you employ the term for features which are not privatively (that is, present/absent, +/−, binary) defined.

For instance: “Hair color (white, blond, red, brown, black) has too many choices and would not be a distinctive feature. But you could pick one distinctive feature—say, “fair+/−” — and have two choices: Blonde (fair+) and Brunette (fair−). Two features, “fair+/−”, “color+/−” would allow you to distinguish four colors: Blond (fair+,color−), Red (fair+,color+), Brown(fair−,color+) and Black(fair−,color−). And so forth.