grounds vs piece of ground

grounds and piece of ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
grounds	Yes	No	No	No
piece of ground	Yes	No	No	No

As nouns, piece of ground is a hypernym of grounds; that is, piece of ground is a word with a broader meaning than grounds:

grounds: a tract of land cleared for some special purposes (recreation or burial etc.)
piece of ground: an extended area of land

Other hypernyms of grounds include parcel, parcel of land, piece of land, tract.

grounds (noun)	piece of ground (noun)
the enclosed land around a house or other building	an extended area of land
your basis for belief or disbelief; knowledge on which to base belief
a justification for something existing or happening
a tract of land cleared for some special purposes (recreation or burial etc.)
dregs consisting of solid particles (especially of coffee) that form a residue