Base

In mathematics, used to refer to a particular mathematical object that is used as a building block. A base-a system is one that uses a as a new unit from which point counting starts again. (See decimal system.) In the mathematical expression an, read as “a to the nth power,” a is the base.

Aviation Maintenance Technician Handbook — General, p. G-2

Also defined in 1 more source

The electrode of a bipolar transistor between the emitter and the collector. Varying a small flow of electrons moving into or out of the base controls a much larger flow of electrons between the emitter and the collector.

Aviation Maintenance Technician Handbook — Airframe, p. G-4

Taught in AF-I Procedures, AF-I Concepts, GEN-H Concepts, PP-A Procedures.