pulley vs idler pulley

pulley and idler pulley both are nouns.

Word	Noun	Adjective	Verb	Adverb
pulley	Yes	No	No	No
idler pulley	Yes	No	No	No

As nouns, idler pulley is a hyponym of pulley; that is, idler pulley is a word with a more specific, narrower meaning than pulley:

pulley: a simple machine consisting of a wheel with a groove in which a rope can run to change the direction or point of application of a force applied to the rope
idler pulley: a pulley on a shaft that presses against a guide belt to guide or tighten it

Other hyponyms of pulley include bollock, bullock block, fairlead, idle pulley, idle wheel.

pulley (noun)	idler pulley (noun)
a simple machine consisting of a wheel with a groove in which a rope can run to change the direction or point of application of a force applied to the rope	a pulley on a shaft that presses against a guide belt to guide or tighten it