A stall is when a moving object's momentum is ceased. A 180 degree turn is when you turn around to face the opposite direction. So a 180 Degree Stall is when the train stops moving in one direction to turn back on its self and go the other way

Very much like a turn around, or a cutback. I've only ever really seen this term used for coasters in regards to Nemesis at Alton Towers in the UK, but maybe it's used generally for inverts or other coasters. I suspect, like many coaster element/inversion names, it's origin is in aviation.
Nemesis' 180 Degree Stall turn
edit: Further reading suggests the upward movement has to do with the "stall". Maybe a pilot would literally stop accelerating and use the upward momentum to turn around? I really don't know enough about this stuff. Hopefully someone knowledgeable will come along and explain more clearly