The whole ensemble is called chut Thai ชุดไทย. The bottom wrap-around, sarong-like thingy with the pantaloon effect is called a chong kraben โจงกระเบน; the top is called different names depending on style and region but the most common one, the Nehru-like jacket with a mandarin collar, is called phraratchathan พระราชทาน. The sash that they tie around the waist is called a pakama ผ้าขาวม้า. There's also a name for the headgear which I can't recall right now (the pakama can also be tied around the head).