Can either be:

1) When someone posts the exact same thing twice in a row.

2) When someone posts something then, rather than using the "edit" button, posts an elaboration or similar right after the 1st.

3) Simply two posts by the same member, regardless of reason, after each other.

v. collectively or individually finding someone/something to blame for a problem, as opposed to brainstorming which connotes finding solutions to a problem

n. the act of finding someone/something to blame for a problem

(from a TV ad for HD television)

1. A double post is two posts in a row that can be made into one post, but the person decides to post it in two separate posts. The posts have to have a common element though, if not it is just rambling.

2. Double(meaning two) posting is two posts in a row, but this is not what Admins and Moderators are really referring to when they say “no double posting allowed on forums.”

Two posts in a row that may or may not have the same meanings.
Complete disregard for the edit button.

Two posts, in succession, in a forum topic created by the same person that say the exact same thing.

When a post is the same as another post by the same member in the same topic.