oblong vs plane figure

oblong and plane figure both are nouns.

Word	Noun	Adjective	Verb	Adverb
oblong	Yes	Yes	No	No
plane figure	Yes	No	No	No

As nouns, plane figure is a hypernym of oblong; that is, plane figure is a word with a broader meaning than oblong:

oblong: a plane figure that deviates from a square or circle due to elongation
plane figure: a two-dimensional shape

Other hypernyms of oblong include two-dimensional figure.

oblong (noun)	plane figure (noun)
a plane figure that deviates from a square or circle due to elongation	a two-dimensional shape

oblong (adjective)	plane figure (adjective)
deviating from a square or circle or sphere by being elongated in one direction
(of a leaf shape) having a somewhat elongated form with approximately parallel sides