<unk> , a variety of anomic aphasia, is a form of aphasia (loss of language capability caused by brain damage) in which the subject has difficulty remembering or recognizing names which the subject should know well.  The subject speaks fluently and grammatically, and has normal comprehension; the only deficit is trouble with "word finding," that is, finding appropriate words for what they mean to say.
Subjects often use circumlocutions (speaking in a roundabout way) in order to express a certain word for which they cannot remember the name.  Sometimes the subject can recall the name when given clues. Sufferers are often frustrated when they know they know the name, but cannot produce it.
Sometimes subjects may know what to do with an object, but still not be able to give a name to the object. A subject is shown an orange, and asked what it is called. The subject may be well aware that the object can be peeled and eaten, and may be able to demonstrate this by actions or even verbal responses.  Whether such a subject could name the color of the orange is unknown.    Responses may differ depending on whether objects are shown in the right or left hand side of the visual field.
Anomia is caused by damage to various parts of the parietal lobe or the temporal lobe of the brain.  This type of phenomenon can be quite complex, and usually involves a breakdown in one or more pathways between regions in the brain.
"Averbia" is a specific type of anomia in which the subject has trouble remembering only verbs.  This is caused by damage to the frontal cortex, in or near Broca's area.
Another type of anomia is "color anomia", where the patient can distinguish between colors but cannot identify them by name.
Nominal aphasia
Global aphasia
Conduction aphasia
Expressive aphasia
Receptive aphasia
Logopenic progressive aphasia
Dysprosody
Hemispatial neglect