The act and process of sorrowing for a death.
The act and process of sorrowing for a death — and its outward signs and customary period.
The outward, social, processual member: where grief and sorrow name the inward feeling, mourning names its expression, observance, and conventions (the act of grieving, the signs like black dress, the defined period — “in mourning,” “national mourning”). The most culturally and ritually loaded term, tied to public observance. Cue: you are “in mourning for” someone; you don't “feel a mourning.” It presupposes a death.
The act of sorrowing; the inner experience and process of grieving a loss.
Distinct from grief (the felt emotion): mourning is its expression and process.
The outward signs and customary period of grief — black dress, armbands, a set mourning period.
The “customary dress” sense dates to the 1650s; the formal mourning period is a social institution.
Old English murnung “complaint, grief, act of lamenting,” from mourn (Old English murnan “to feel or express sorrow; be anxious”), from Proto-Germanic *murnan “to remember sorrowfully,” probably from PIE *(s)mer- “to remember” (or *mer- “to die, wither”).
Developed from general “expression of grief” toward the social/ritual senses: the outward-sign sense (“customary dress”) from the 1650s, and the “display the conventional appearance of grieving for a period” sense from the 1520s. No reliable recent-generation shift.