Definition of "fast"
fast1
/fɑːst/
adjective
comparative faster, superlative fastest
Moving with great speed, or capable of doing so; swift, rapid.
(nuclear physics, of a neutron) Having a kinetic energy between 1 million and 20 million electron volts; often used to describe the energy state of free neutrons at the moment of their release by a nuclear fission or nuclear fusion reaction (i.e., before the neutrons have been slowed down by anything).
adverb
comparative faster, superlative fastest
fast2
/fɑːst/
verb
third-person singular simple present fasts, present participle fasting, simple past and past participle fasted
(intransitive) To practice religious abstinence, especially from food.