mourning band; mourning ribbon
A black ribbon or armband worn to express mourning, often at funerals or memorial events.
葬儀では、参列者が喪章を腕に付けていた。
At the funeral, attendees wore mourning bands on their arms.
He attended the funeral wearing a mourning band.
The attendees had mourning bands wrapped around their arms.
腕章 is a generic armband worn for any purpose (duty, identification, etc.), whereas 喪章 specifically denotes the black armband or ribbon worn to express mourning.
Compound of 喪 (mo, 'mourning') and 章 (shō, 'badge' or 'emblem').