Yum is the sound you make when food tastes good, used as a word. It is a whole sentence by itself — you say it and stop. Someone puts a plate in front of you, you look at it, you say "Yum!" You take the first bite and say it again.

It is very informal and slightly childlike. Adults do say it, especially in relaxed company or writing to friends, but a teacher would not write it in an essay. Children say it constantly, and parents say it back to them, sometimes stretched out: "Yum yum!" You will also see it doubled and hyphenated in the fixed phrase yum-yum. Online it is everywhere — under photos of food, as a comment on its own, often just "yum" with no capital letter and no punctuation.

Two useful stretches. First, English lets people say it for things they merely want, not only taste: someone posting a picture of a beach might joke "yum," and people also say it about an attractive person, which can be flirty or rude depending on who says it to whom. Be careful there. Second, its opposite is yuck (or eww) — that pair, yum and yuck, is one of the first things English-speaking children learn to sort the world with.

The word is imitative: it copies the noise of enjoying a mouthful, the same way "boom" copies an explosion. That is why every language has something like it and none of them quite match.