Average is a related term of plain.
As nouns the difference between average and plain
is that
average is (legal|marine) financial loss due to damage to transported goods; compensation for damage or loss while
plain is (rare|poetic) a lamentation or
plain can be an expanse of land with relatively low relief.
As adjectives the difference between average and plain
is that
average is (not comparable) constituting or relating to the average while
plain is .
As verbs the difference between average and plain
is that
average is (informal) to compute the arithmetic mean of while
plain is to lament, bewail or
plain can be (obsolete|transitive) to plane or level; to make plain or even on the surface.
As an adverb plain is
(colloquial) simply.