What is an emotion, and why can't scientists agree on one definition?

An emotion is a state of body and mind, set off by something that matters to you, that mixes a feeling, a bodily change and often an urge to act. Scientists use the word all the time, but there is no scientific consensus on a single definition. That is the first surprise about the field.

Here is why it is hard. An emotion is not one thing. It has several components: the subjective experience (what it feels like), cognitive processes (how you size up the situation), expressive behavior (your face, your voice), changes in your body, and behavior aimed at doing something about it. At one time researchers each pinned emotion on one of these parts. William James pointed to the felt experience, behaviorists to behavior, and physiologists to bodily changes. More recently, emotion has been said to consist of all of them together.

Emotions come in different sizes, too. They can be occurrences, like panic, or lasting dispositions, like hostility. They can be short-lived, like anger, or long-lived, like grief. And they are often intertwined with mood, temperament and personality, which is part of why the edges blur.

The word itself has a short history. It comes from Latin, where it meant a movement from the inside towards the outside, and the French humanist Étienne Dolet coined it in 1543. The modern idea is younger still. One historian's line is that no one felt emotions before about 1830: people felt passions, accidents of the soul and moral sentiments, and explained them quite differently.