match against an outside team; interschool game
Used in sports contexts for a game against a team from another school or organization, as opposed to an intra-squad practice.
We are practicing every day for the interschool match next week.
I played against athletes from another school for the first time in an outside match.
練習試合 is a practice match, which can be against an outside team or within the same team, while 対外試合 specifically emphasizes the opponent is from outside the group.
紅白戦 is an intra-squad scrimmage (often split into red and white teams), the opposite of 対外試合.
Compound of 対外 (taigai, 'outside, external') and 試合 (jiai, 'match, game').