crabapple

A small, rosy red apple with a rather hard, extremely tart flesh. Crabapples, available during the fall months, are too sour for out-of-hand eating but make outstanding jellies and jams. Spiced and canned whole, they're a delicious accompaniment for meats such as pork and poultry. See also apple.

The noun has 3 meanings:

Meaning #1: any of numerous varieties of crab apples cultivated for their small acid (usually bright red) fruit used for preserving or as ornamentals for their blossoms
  Synonyms: crab apple, cultivated crab apple

Meaning #2: any of numerous wild apple trees usually with small acid fruit
  Synonyms: wild apple, crab apple

Meaning #3: small sour apple; suitable for preserving
  Synonym: crab apple