General:
* Fold
* Crease
* Tuck
* Gather
* Ruff
* Flute
* Gathers
Specific to fabric:
* Box pleat
* Knife pleat
* Accordion pleat
* Pinch pleat
* Kick pleat
The best synonym will depend on the specific context. For example, if you are talking about the pleats in a skirt, you might use the word "gather" or "tuck". But if you are talking about the pleats in a piece of fabric, you might use the word "fold" or "crease".