To /Open or /Raise while holding a hand that is likely to be inferior, hoping that all other players will /Fold and award you the pot without a /Showdown.
In games with multiple betting rounds, to bluff on one round with an inferior hand that might become a much better one by chance in a later round is often called a semi-bluff. For example, a player in a stud poker game with four spade-suited cards showing (but none among his downcards) on the penultimate round might raise, hoping that others believe he has a /Flush even though he doesn't. If his bluff fails and he is called, he still might be dealt a spade on the final card and win the showdown (or he might be dealt another non-spade and try his bluff again, in which case it is a pure bluff rather than a semi-bluff.