Finding the rate of change, or derivative, of a function — from the power, chain, and product rules through partial derivatives and gradient descent to the Fréchet derivative, which extends the idea to infinite-dimensional spaces.
Differentiation means substantially different things in different fields, and nothing resolves the ambiguity but context. In mathematics it is the operation of finding a derivative, a rate of change, with a settled formal treatment. In cell biology it is the process by which a less specialised cell acquires a specialised character and function. In business strategy it is the positioning of an offering as distinct from competitors on grounds other than price. In sociology it refers to the increasing specialisation of institutions and roles within a society. None of these senses derives from the others in any way useful to a reader, and no authority governs the word across them; which sense this lexicon uses is a choice stated openly rather than defended by a citation that could only cover one field.
Why this entry carries no source list. This lexicon cites an official primary source wherever one exists, and says so plainly where none does. No body governs this term. There is no standards organisation for it, no register, and no text whose authors could claim to have settled its meaning. Methods literature describes practice and argues about it; a citation to one author would dress a position in a live argument as a rule, which would mislead more than silence does. What follows is the convention as it is actually used, with the disagreements named rather than smoothed over.