From Memetics

(same as Bait, opposite of Threat)

The part of a meme-complex that urges replication. The hook is often most effective when it is not an explicit statement, but a logical consequence of the memes content. (Hofstadter)

After a host has been located using the bait, the hook is the co-meme that then results in replication of the meme. The term is from Hofstadter.