From Online Manual
A post, often called a message or reply, may be either a reply to a topic or the topics first post (often called the original or opening post).
The word “post” is analogous to making a note on a physical bulletin board, where you might call the note a "post". Sometimes, the words "topic" or "sticky" may refer only to the first post and not the replies.
Posts are made on the boards and topics within a forum by guests and or members. Posts should not be confused with personal messages.
By default, you are limited to having approximately 4,294,967,296 posts. Realistically it may not be practical or possible for some to reach this limit.