an army; a military force
Refers to a single organized army or military unit, often in historical or strategic contexts.
He led an army and advanced into enemy territory.
the whole army; the entire force
Emphasizes the entirety of a military force, not just a part of it.
一軍が総攻撃を開始した。
The whole army launched an all-out attack.
first team; top team; first-string players
Sports term for the primary or varsity squad, as opposed to the second team (二軍).
See also: 二軍
彼は今年から一軍に昇格した。
He was promoted to the first team this year.
the popular group; the cool kids
Colloquial use, often in school settings, referring to the socially dominant or popular clique.
彼女はクラスの一軍グループにいる。
She's in the popular group in class.
He was promoted from the second team to the first team last month.
The ones in the class's popular group are the students who stand out.
He got dropped from the first team due to an injury.