When I was a trucker I heard an old boy from down-yonder talkn' on the CB radio and along with lots of other folksy patois he called thermal underwear "longjeans".  I had to LMFAO, but I wouldn't call them any other way now. And DuoFold 410b is a good dual layer "longjean" great for standing around in very cold weather to moderate exertion. During a recent cold snap here in Wisconsin we had daytime temperatures around 0.  I worked outside with these babies under regular jeans and wasn't cold at all (except finger tips).  For sweaty high exertion, aerobic sports like skiing though, I'd look for something synthetic, which duofold also makes.