In ballroom dancing, turns are typically described as fractions of a full 360 degree rotation. A half-turn is therefore simply an "about face" -- a 180 degree turn. When you start facing north and dance 1/2 turn, you will finish facing south.

One will usually also specify the direction of rotation -- to the left (counter-clockwise) or to the right (clockwise). Begin facing north and dance dance 1/4 turn (90 degrees) to your right (clockwise), and you will end facing east.

I hope that makes sense!

Regards,
Jonathan