1. of only ordinary or moderate quality; neither good nor bad; barely adequate: The car gets only mediocre mileage, but it's fun to drive. Synonyms: undistinguished, commonplace, pedestrian, everyday; run-of-the-mill. Antonyms: extraordinary, superior, uncommon, incomparable.
2. not satisfactory; poor; inferior: Mediocre construction makes that building dangerous. Synonyms: meager, low-quality, second-rate; so-so. Antonyms: excellent, superior.
Accurate definition courtesy of dictionary.com.