I've had trainers use the terms sidepass and half-pass interchangeably but no matter which they used it was the same way..head bent in the direction of travel. Admittedly these aren't specific dressage trainers just trainers doing dressage type flatwork on horses that were most definitely not dressage capable :).
After looking it up, the half-pass is where the head is bent in the direction of travel as the horse moves both sideways and forward. In the sidepass, the head is straight and the horse isn't necessarily moving forward..in other words, a true "side to side". In the leg yield, the head is bent in the opposite direction of travel as the horse is moving sideways and forward.
Here is the site I used just now to find the differences: Leg Yield, Half Pass & Side Pass