crosspost

Short definition from Wiktionary

Noun

1. 1
   An electronic message posted to multiple newsgroups or forums simultaneously.
2. 2
   An instance of crossposting, or the post thus created.

Verb

1. 1
   To post (a message) to multiple newsgroups or forums simultaneously.
2. 2
   To post an entry (in a book index or other index) such that no matter which word a user looks under, they will find the relevant entry (see examples).