piece vs lump

piece and lump both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece	Yes	No	Yes	No
lump	Yes	No	Yes	No

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

Other hyponyms of piece include body part, corpus, acicula, base, corner, craton, cutting, fragment, hunk, nub, stub, segment, slice, strip, world.

piece (noun)	lump (noun)
a separate part of a whole	a compact mass
game equipment consisting of an object used in playing certain board games	a large piece of something without definite shape
a portable gun	an awkward stupid person
a work of art of some artistic value	an abnormal protuberance or localized enlargement
an item that is an instance of some type
a distance
a musical work that has been created
an artistic or literary composition
an instance of some kind
a serving that has been cut from a larger portion
a portion of a natural object
a share of something
a period of indeterminate length (usually short) marked by some action or condition

piece (verb)	lump (verb)
repair by adding pieces	group or chunk together in a certain order or place side by side
eat intermittently; take small bites of	put together indiscriminately
join during spinning
to join or unite the pieces of
create by putting components or members together