Real Dictionary

bitter
A bitter experience makes you feel very disappointed. You can also use bitter to emphasize feelings of disappointment.
adjective: A bitter taste is sharp, not sweet, and often slightly unpleasant.
adjective: Bitter weather, or a bitter wind, is extremely cold.
adjective: If someone is bitter after a disappointing experience or after being treated unfairly, they continue to feel angry about it.
adjective: In a bitter argument or conflict, people argue very angrily or fight very fiercely.
variable noun: Bitter is a kind of beer that is light brown in colour.

bitter in American English
designating or having a sharp, often unpleasant taste; acrid, as quinine or peach stones
adjective: having a harsh, disagreeably acrid taste, like that of aspirin, quinine, wormwood, or aloes
adverb: extremely; very; exceedingly
noun: that which is bitter; bitterness
transitive verb: to make bitter

bitter in British English
adjective: having or denoting an unpalatable harsh taste, as the peel of an orange or coffee dregs
adverb: very; extremely (esp in the phrase bitter cold)
noun: a thing that is bitter
verb: to make or become bitter