Acronym of post office preferred. (denoting a standard envelope size)

(countable) A loud, sharp sound, as of a cork coming out of a bottle. / (uncountable, regional, Midwestern US, Canada, Britain) An effervescent or fizzy drink, most frequently nonalcoholic; soda pop.

(intransitive) To make a pop, or sharp, quick sound. / (ergative) To burst (something) with a popping sound.

Used to represent a loud, sharp sound, as of a cork coming out of a bottle.

(colloquial, endearing) One's father.

(used attributively in set phrases) Popular.

Pop music.

(Russian Orthodoxy, uncommon) A Russian Orthodox priest; a parson.

A social club and debating society at Eton College. / The body of college prefects.

(also in plural) A popular classical music concert.