顿

顿 (dùn) is most commonly a measure word (classifier) for meals and, by extension, for bouts of scolding, beating, or persuasion — anything that happens as one 'session': 一顿饭 ('a meal'), 三顿饭 ('three meals a day'), 骂了一顿 ('gave a good scolding'), 打了一顿 ('gave a beating'). This bout sense is what surprises English speakers: 顿 counts an episode of an action, often a heavy or emotional one, where English would say 'a good telling-off' or 'a round of.' As a verb, 顿 means 'to pause' or 'to stop briefly' (停顿 tíngdùn, 'to pause'), and it appears in 顿时 (dùnshí, 'suddenly / at once').

It is neutral in register. When counting meals it is the default and only natural classifier — you always say 一顿饭, never 一个饭.