En croix is a classical ballet term meaning “in the shape of a cross.” This term is usually used in ballet class and lets a dancer know the step should be done to the front, side and then back. Doing steps en croix can also be done in reverse where they start from the back, side, then front.
For example, a teacher may ask that you do tendu en croix. This would usually mean that you would do four tendus: one front, one side, one back, and one side again. They can also be done in a series, so possibly eight tendus in a row, but done en croix, so the direction changes every time. En croix is commonly given during barre for a well rounded exercise, but also can be done in the center.