In a situation which is entirely suitable, familiar, or enjoyable.

(idiomatic) First; foremost; best, often used after its headword.

The most important person, the one who is in charge. / Someone who is top of a ranking, who is ranked first. / Oneself, being considered foremost, as by an egoist.

(In both the 12-hour clock and the 24-hour clock) The start of the second hour of the day; 1:00 a.m. (01:00). / (Only in the 12-hour clock) The start of the fourteenth hour of the day; 1:00 p.m. (13:00).

The very beginning. / (US, slang) A close, longtime friend, one who has been around since the beginning.

At the very beginning; when something is entirely new.

Alternative form of all as one

(soccer) A quick one-touch play between two players. / (boxing) A one-two punch; two punches delivered in rapid succession / Two things in short succession

A unique specimen.

The cardinal number occurring after do and before do two in a duodecimal system. Written 11, decimal value 13.