A reply is a pulic response to a post created inside a community, in contrast to a direct message, which is private. Depending on the platform, a reply can start or continue a comment thread, in which one or more users comment on a post or engage in discussion about it. Some platforms employ hierarchies which enable multiple threads and sub-threads to spawn from a single post, while others keep all replies in a single thread for the sake of simplicity.

The length of any given reply depends on the user creating it and the character limits imposed by the relevant platform.