Aphasia is caused by localized brain damage, for example due to a stroke or an automobile accident. General intellectual functioning is not necessarily impaired, as the person can still perform non-linguistic tasks. Nor is the understanding and production of language necessarily completely abolished. Instead, there are highly specific patterns of impairment in the way language is processed.

Humans possess a general language capacity that allows them to speak particular world languages. Language is highly complex but can be defined in terms of a number of key properties.

There are several ways of defining words: as they appear in writing, in dictionaries or in grammar. Typically three word groups are recognised: lexical words, function words and inserts. This article defines each group.