What do the letters in B A S E jumping stand for?

BASE is an acronym for Building, Antenna, Span, and Earth. The term was coined by Carl Boenish with reference to the four different types of structures from which one could jump.

Building: Jumping from the top of a building.

Antenna: Jumping from a tall radio or TV antenna.

Span: Jumping from a cable car over a gorge or river.

Earth: Jumping from a cliff or natural object such as El Capitan.