2 questions - one about collusion the other about a definition

[Gonso]

Opening usually refers to the first person to raise preflop (technically you can't bet preflop because of the blinds). That's called an open-raise. You can also open-limp.

I've actually heard the term open-fold by a couple guys on cardrunners, but that sounds a little goofy to me.

The collusion thing: no, you can't agree to check a hand down during a hand. You also can't communicate any kind of agreement to cooperate against another player for later hands. It's common strategy to check down hands to eliminate a player, fine, but you just can't do it out loud.