One-Plane
The "one" refers to a single plane that is made by the shoulders and the left arm(for a right-handed golfer). A true one-planer will have their left arm fairly parallel to line drawn from shoulder to shoulder at the top of their backswing.

As you can see in this picture, this golfer is wrapping his arms around his body rather than lifting them up, a clear indicator of a one plane swing. Notice how the plane of his left arm is virtually identical to the plane that his shoulders are turning on(designated by the redline). Hence the term "one"-plane because the shoulders and arms are moving along a single plane without any separation.