Forme fruste (rare plural: formes frustes) is a term which refers to the incomplete phenotypic expression of a condition, such that it does not meet the usual diagnostic criteria.
For example, an individual with imaging features consistent with tuberous sclerosis, but without skin lesions, seizures or intellectual disability would be said to have a forme fruste of the condition. The term is vague as, by definition, it has no clear diagnostic criteria .
History and etymology
The term is French, meaning "defaced form" .