1.nounA person of minor significance, accomplishment or acclaim; a common and undistinguished person.
2.nounA member of a socioeconomic class between the upper ranks of society and the agricultural workers.
3.adjectiveHaving no peculiar or outstanding features; not extraordinary, special, exceptional, or great; of medium quality, almost always with a negative connotation.
"I'm pretty good at tennis but only mediocre at racquetball."