Co-op is a cut game mode from Black Mesa.
"Black Mesa co-op will allow players to not only recruit the assistance of security guards and scientists, but also that of your friends! Share the single-player experience by battling through Black Mesa together. Players will be able to wage battles, travel corridors and emerge victorious over the enemies and escape the confines of the Black Mesa Research Facility."
On March the 17th 2010, it was announced that Co-op would no longer be featured in Black Mesa. Due to the great amount of problems to port and make Co-op code working, the developers decided to stop its development and focus only in Single-Player and Deathmatch. 
Co-op could potentially arrive after the release of SP, but would not be made by the Black Mesa official team.  It has also been stated that since dropping the Co-op mode, Black Mesa's chapters have been redesigned with only single-player activity in mind, and that the new "level design and gameplay don't lend themselves well to a co-op experience."