From Old English 'blōstm,' PIE *bʰleh₃- (to blow, to bloom) — kin to 'bloom' and 'flower,' taking the Germanic path with a *-st- suffix.
A flower or mass of flowers, especially on a fruit tree or ornamental plant; the state or time of flowering.
From Old English 'blostm' or 'blostma' (flower, blossom), from Proto-Germanic *blostmaz, derived from the PIE root *bhleh3- (to blow, to bloom, to flourish) with an additional formative suffix *-st-. This PIE root is richly attested: it produced Latin 'flos, floris' (flower — floral, flourish, flour), Old English 'blowan' (to bloom — used in 'the roses blow'), and Proto-Germanic *blomo (bloom). The suffix *-st- distinguishing 'blossom' from bare 'bloom' may have originally marked
In English, 'blossom' and 'bloom' have quietly divided the botanical territory: 'blossom' is conventionally reserved for the flowers of fruit trees (cherry blossom, apple blossom), while 'bloom' applies to ornamental flowers (rose bloom, lily bloom). No grammarian decreed this — it emerged organically from centuries of usage.