figure 8 vs plane figure

figure 8 and plane figure both are nouns.

Word	Noun	Adjective	Verb	Adverb
figure 8	Yes	No	No	No
plane figure	Yes	No	No	No

As nouns, plane figure is a hypernym of figure 8; that is, plane figure is a word with a broader meaning than figure 8:

Other hypernyms of figure 8 include two-dimensional figure.

figure 8 (noun)	plane figure (noun)
a two-dimensional figure having the shape of the number eight	a two-dimensional shape