to laugh, and by extension to play, celebrate, or mock; the whole range of glad, playful, or scornful laughter.
At its center this word is the physical act of laughing, and everything else grows out of it. The Bible uses it for four related things, and the same form can shade from one to the next depending on who is laughing and at whom.
The warm end is genuine gladness: a person laughs out of delight, or laughs in the face of hard times because they are unafraid. From there the word widens to play and sport, the way children play in the street, animals frolic in a field, or people amuse themselves, sometimes in innocent fun and sometimes as a cover for cruelty or deceit. It widens again to full-scale celebration: making merry with singing, dancing, and instruments, as when David danced before the ark.
The other end is the opposite of warmth. To laugh at someone can mean to look down on them and jeer. This is the laughter of scorn, and it is what God is pictured doing toward defiant rulers who imagine they can resist him: he laughs at them, not because it is funny, but because their revolt is empty. Which sense is in play is almost always clear from the setting: a small word like at often turns a laugh of joy into a laugh of contempt.
Hebrew has a near-twin of this word, spelled with a slightly different opening sound, that carries the very same range of meanings. The two are so close that they are best thought of as two spellings of one idea.