Mirth, n.
Etymology: [OE. mirthe, murthe, merthe, AS. myredh, myrgedh, merhedh, mirhedh. See Merry.]
1. Merriment; gayety accompanied with laughter; jollity. Then will I cause to cease ... from the streets of Jerusalem, the voice of mirth. Jer. vii. 34.
2. That which causes merriment. [Obs.] Shak.
Syn. -- Merriment; joyousness; gladness; fun; frolic; glee; hilarity; festivity; jollity. See Gladness.
Shades of meaning
Fernald has no entry of its own for mirth; he lists it under happiness.
He lists it without comment; the entries above explain its neighbors.