A figure of speech is a word or phrase that is not intended to be interpreted literally. Figures of speech are literary devices intended to make language more interesting, intense or colorful. Because a figure of speech is not meant to be understood literally, it complicate decoding by both humans and artificial intelligence (AI) software systems that use natural language understanding (NLU). Language processing software requires word sense disambiguation (WSD) to reliably interpret figures of speech and other examples of linguistic ambiguity.
Figures of speech are one type of figurative language, which also includes imagery and sound-based enhancements such rhyming schemes or onomatopoeia. An entire piece of content, such as a poem or a story, may be spoken of as figurative language. In contrast, a figure of speech is usually presented in a group of words no larger than a sentence.
Idioms are probably what is most commonly thought of as figures of speech. An idiom is a phrase or sentence that means something different from what a literal interpretation would suggest. Most native English-speakers can interpret the idiomatic sentence, “It never rains but it pours,” to mean that events, such as misfortunes, seem to happen in clusters. To an AI program or a non-native speaker, however, that sentence could be very difficult to decipher because its two parts, if taken literally, are self-contradictory: It never rains | it pours.
Irony, another kind of figure of speech, can only be understood in context. Verbal irony often means that the literal meaning of a statement is completely reversed. For example, someone handed an emergency assignment on Friday afternoon might say, “Great! I was wondering what I was going to do this weekend.” A human might immediately understand that as ironic, but a software program would need to understand the rules of irony and have some knowledge of when it was likely to apply.
Metaphor is another type of figure of speech, in which one thing is represented by a different word with descriptive power, as in “the moon was a ghostly galleon.” Simile, another type, likens one thing to another, as in “the moon was floating like a balloon.” Other examples of figure of speech include hyperbole (exaggeration), euphemism (substituting mild terms for blunt ones) and personification (speaking of non-human entities as if they were people).