collar vs leash

collar and leash both are nouns.

Word	Noun	Adjective	Verb	Adverb
collar	Yes	No	Yes	No
leash	Yes	No	Yes	No

As nouns, collar and leash are synonyms defined as:

collar (noun)	leash (noun)
the act of apprehending (especially apprehending a criminal)	a figurative restraint
a figurative restraint	restraint consisting of a rope (or light chain) used to restrain an animal
a band that fits around the neck and is usually folded over	the cardinal number that is the sum of one and one and one
necklace that fits tightly around a woman's neck
a band of leather or rope that is placed around an animal's neck as a harness or to identify it
the stitching that forms the rim of a shoe or boot
a short ring fastened over a rod or shaft to limit, guide, or secure a machine part
anything worn or placed about the neck
(zoology) an encircling band or marking around the neck of any animal