Gladness expressed in laughter.
Gladness expressed in laughter — the merriment of jokes, comedy, and convivial company.
Defined by its tie to laughter and amusement; it is the joy of humor and conviviality, and unlike glee it is never malicious. Outward, social, and transient rather than an inner settled state. Addison's classic contrast: mirth is “short and transient,” a flash, whereas cheerfulness is fixed and permanent. More about fun and humor than the quiet, sensory pleasure of delight.
Old English myrgð “joy, pleasure” (also “eternal bliss”), from the root of merry + -th; merry ultimately traces to a PIE root meaning “short,” via the idea of making time seem to fly.
Narrowed and secularized: the Old English religious senses (“eternal bliss, salvation”) died out, leaving the laughter/merriment sense. No sourced recent-generation shift.