A message with its replies in a newsgroup is often called a

A thread is an original message together with all the replies posted in response to it, grouped together in a newsgroup or forum.

A list is merely a series of items and does not capture the reply structure of a discussion.

A comment is a single response, not the whole chain of a message and its answers.

A post is one individual message, while the entire conversation built from it is more than that.

Threading keeps related exchanges organised so readers can follow a discussion.

Email clients use the same idea to group messages sharing a subject.