A nonsense word is a syllable or group of syllables that can pronounced based on the phonetic rules of a language but which transmit no meaning to a reader or listener. A word such as "Keev," for instance, is not a real word in the English language but can still be pronounced based on the phonetic rules of English. Such words appear in a variety of literary contexts but generally only exist because of the sound of the nonsense word. "Keev," for example, rhymes with "leave" and may be used in a poem for that reason, particularly if a meaningless sound is needed in place of any word with real meaning.
Literature and poetry are one common context in which a nonsense word may be used. Sometimes, as when seeking a rhyming word, a writer may use a nonsense word because of the particular sound that it makes. In verse, a writer may also choose to use a word with stressed or unstressed syllables based on the meter of a poem. Such words are sometimes used to create humor in literature as well.
Over time, a nonsense word may come to take on meaning because it is used for some particular purpose until it becomes accepted into the language's lexicon. In other cases, ascribing meaning to a nonsense word may be a completely intentional and premeditated act. This can, for example, occur when a scientific discovery is made and needs a name.
In some cases, one or more nonsense words are given specific meaning or used for a specific purpose without becoming a part of the language's lexicon. Words used in this manner are known as "nonce words." Such words are, in general, only used once and in one context, as in a specific book or poem by one writer. The single use of a word is generally not sufficient to earn the word a place in a language's dictionaries.
Individuals with aphasia, a disorder resulting from damage to the language sections of the brain, sometimes mix in a nonsense word or nonsense words with their speech. This occurs specifically in expressive aphasia, which affects the manner in which an individual communicates. These nonsense words are sometimes mixed in with real words in a kind of "word salad" that contains no real overall meaning. Depending on the type of aphasia the patient has, he may or may not be able to understand spoken and written language.