Ambiguity

Ambiguity: of words or phrases is the ability to express more than one interpretation. It is distinct from vagueness, which is a statement about the lack of precision contained or available in the information.

-from http://en.wikipedia.org/wiki/Ambiguity

In other words, ambiguity is something that is up for interpretation because the intention is not clear. This should be avoided because it leaves the reader confused on the meaning. Usually when ambiguous language is used, there is only one intentional meaning to be expressed. They did not realize the other possible meaning.

Some examples are:

Indian folk lore researcher..

is it [Indian] folk lore researcher or [Indian folk lore] researcher

The short man and woman stood next to each other..

is it the short man and short woman or only short man

However, ambiguity can also be a useful tool for humor as a play on words.

Some examples are:

"Don’t It Make My Brown Eyes Blue" by Crystal Gayle

Blue can mean the feeling of sadness or literally mean her brown eyes changed to the color blue

"Last night I shot an elephant in my pajamas. How he got in my pajamas, I’ll never know" a joke by Groucho Marx

The 1st sentence, readers would conclude he shot the elephant while wearing pajamas. The 2nd line is the joke.