appearances; what others think; public decency
Refers to how one is perceived by society, often with a focus on maintaining a respectable image. Commonly used in phrases like 世間体を気にする (to care about appearances) or 世間体が悪い (to look bad in public).
He doesn't wear flashy clothes because he cares about what others think.
It would look bad in the eyes of society if you did that.