What is meant by the term source?

Answer 1

usually power source

where power comes from

logic gates outputs are commonly called source or sink

say the power source to the package is positive (VCC is +5)

to call the output a source we would use an NPN transistor

with the collector connected to +5V

the base goes to the logic the emitter is the output terminal

the output is the source of +5v

sink output would have the emitter grounded

base to logic

the collector is the output

so the output will sink positive voltage to ground

A BUFFER usually has two output transistors one for source one for sink and can do either or both