The main or first party of the night (e.g., the first party after an event), often followed by a second party (二次会).
会社の歓迎会は一次会だけ参加して帰ります。
I'll only attend the first party at the company's welcome party and then go home.
結婚式の後、一次会は親族だけで行った。
After the wedding ceremony, the first party was held only with relatives.
一次会でお開きになったが、数人で二次会に行った。
The first party ended, but a few of us went to the second party.
一次会の会場は駅前の居酒屋だった。
The venue for the first party was an izakaya in front of the station.