As the first in a number of examples or reasons. / As a single given example, often the most obvious or important one and almost certainly enough to sway the argument, of several reasons for something.

simple past tense and past participle of one

(collectible card games) A card included in a deck only once.

(formal) simultaneously, at the same time, all together / (chiefly UK, formal) in agreement

(UK, dated, informal) To be amusing in a slightly challenging way.

In a single attempt. / (theater) Performed downstage between the frontmost pieces of scenery.

simple past tense of be a one

Third-person singular simple present indicative form of be a one

The cardinal number one hundred plus one.

(slang, US) information