Definition:

The multiple meanings, either inentional or unintentional, of a word, phrase, sentence, or passage.

Examples:

“Good,” for example, can mean “useful” or “functional” (That’s a good hammer)
“exemplary” (She’s a good student)
“pleasing” (This is good soup)
“moral” (He is a good person)

source: http://en.wikipedia.org/wiki/Ambiguity