This article is from the Ballet and Modern Dance FAQ, by Tom Parsons email@example.com with numerous contributions by others.
B-plus is "crois'ee derri`ere", usually used to describe the
position you take before doing a combination. You stand in "crois'ee" with
the working leg to the back, relaxed and slightly bent but ready to move
when the combination begins. The term originated in the New York City
Ballet, and the "B" apparently stands for Balanchine.