means The sound and physical act of expressing amusement, joy, or relief through a series of involuntary vocal bursts.

from From Old English 'hleahtor,' itself rooted in the verb 'hliehhan,' to laugh. The whole family is wonderfully echoic — the word is basically the sound itself, frozen into letters. It has cousins all across the Germanic tongues (German 'Gelächter,' Old Norse 'hlátr'), each one a slightly different transcription of the same noise humans have been making since long before they had grammar to interrupt it.