If you see that a hot-air balloon is rising above the top of a hill, what is the top of the hill called?

reference point

i forgot where i learned this but for some weird reason its still in my head

The top of the hill, when referred to in the context of a hot-air balloon rising above it, is called a "launch site" or a "take-off site." It is the location from where the hot-air balloon is launched into the air.