A word, phrase, sentence, or other communication is called ambiguous if it can be reasonably interpreted in more than one way. The simplest case is a single word with more than one sense: The word "bank", for example, which can mean "financial institution", "edge of a river", or other things. Sometimes this is not a serious problem because a word that is ambiguous in isolation is often clear in context. Someone who says "I deposited $100 in the bank" is unlikely to mean that he buried the money beside a river. More problematic are words whose senses express closely related concepts. "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" (She is a good person), and probably other similar things. "I have a good son" isn't clear about which sense is intended.
Ambiguity should not be confused with vagueness, in which a word or phrase has one meaning whose boundaries are not sharply defined.
In addition to words with multiple senses, ambiguity can be caused by syntax. "He ate the cookies on the couch", for example, could mean that he ate those cookies which were on the couch (as opposed to those that were on the table), or it could mean that he was sitting on the couch when he ate the cookies. Spoken language can also contain lexical ambiguities, where there is more than one way to break up a set of sounds into words, for example "ice cream" and "I scream". This is rarely a problem due to the use of context.
Philosophers (and other users of logic) spend a lot of time and effort searching for and removing ambiguity in arguments, because it can lead to incorrect conclusions and can be used to deliberately conceal bad arguments. For example, a politician might say "I oppose taxes which hinder economic growth". Some will think he opposes taxes in general because they hinder economic growth; others will think he only opposes those taxes that he believes will hinder economic growth. The politician hopes that each will interpret the statement in the way he wants, and both will think the politician is on his side. The logical fallacies of amphiboly and equivocation also rely on the use of ambiguous words and phrases.
In literature and rhetoric, on the other hand, ambiguity can be a useful tool. Groucho Marx's classic joke depends on a grammatical ambiguity for its humor, for example: Last night I shot an elephant in my pajamas. What he was doing in my pajamas I'll never know. Songs and poetry often rely on ambiguous words for artistic effect, as in the song title "Don't It Make My Brown Eyes Blue" (where "blue" can refer to the color, or to sadness).