Emulsion
Etymology
From Latin "emulgere" (to milk out), from "e-" (out) plus "mulgere" (to milk) — the word was coined because milk itself is the archetypal emulsion, fat droplets suspended throughout water.
noun · /ih-MUL-shun/
A stable mixture of two liquids that normally refuse to combine, such as oil and water, held together in tiny suspended droplets.
What it names
It explains why mayonnaise, vinaigrette, hollandaise, and butter sauces hold their creamy body — and why they “break” back into oily puddles when they fail.
Example
Whisking oil into the egg yolk drop by drop coaxes the mixture into a thick emulsion instead of a greasy split mess.
Related idea
Related to reduce: a reduction thickens by removing water, while an emulsion thickens by dispersing fat — two different routes to a rich sauce.