Ambiguity happens when sentences or smaller fragments of text are susceptible of interpretation in more than one way. Computer languages are typically designed so as to avoid ambiguity, but human languages seem to be riddled with situations where the listener has to choose between multiple interpretations. In these cases we say that the listener resolves the ambiguity. For human beings the process of resolution is often unconscious, to the point that it is sometimes difficult even to recognise that there ever was any ambiguity. For psychologists ambiguity is an enormously useful tool for probing the behaviour of the human language faculty, while for computational linguists it is usually an unwelcome problem which must be dealt with by fair means or foul.
There are several ways in ambiguity can arise: