armband
n. countable B2 Upper Intermediate US //ˈɑɹmˌbænd// UK //ˈɑːmbænd// arm·band
n. a piece of cloth or plastic worn around the upper arm. It is often used to show a person's role in a group, like a leader or a referee.
n. a strip of fabric or plastic worn around the upper arm, typically to indicate a person's status, role, or team affiliation.
The referee wore a bright yellow armband to show his position.
The team captain was easily identified by the red armband he wore during the championship match.
In many professional leagues, the captain is distinguished by a black armband, a tradition that serves both as a visual identifier and a mark of leadership.
From arm + band.