An armband is a band of fabric that you wear round your upper arm in order to show that you have an official position or belong to a particular group. Some people also wear a black armband to show that a friend or relation has died.
N-COUNT
（用于学游泳的）充气臂圈，救生臂环
Armbands are plastic rings filled with air that people who are learning to swim wear on their upper arms to help them float.