When patients present with medically unexplained symptoms, the aetiology is most often not monocausal but rather multifactorial, and can be better understood through the biopsychosocial model which takes into account the complex interplay of biological, psychological and social factors. 1 However, symptoms are frequently attributed monocausally to an abnormal occlusion, faulty dentures or temporomandibular joint findings. 2 Curiously, a linguistic obstacle may contribute to this understanding (or misunderstanding): the ambiguity of the word FUNCTIONAL.The ambiguity of words is common in everyday language.Examples are the words bright, bank, wing, run-they are called polysemes. In many cases, there are between the different uses of a word. 3 An example of this is the polysemous adjective hard:Hard-hearted, Hard life, Hard material, Hard water.The first two expressions show a certain affinity with the third expression (hard material). They can be described as metaphorical transfers. In the fourth example (hard water), on the contrary, there is no clear connection with the other usages. 'Such expressions presuppose knowledge of technical language for their interpretation or must be regarded as conventionalized in technical language'. 4 It should therefore be noted that the use of polysemous adjectives in technical language, as in medical language, is generally not intuitively comprehensible; their meaning must be learned.In dentistry, there are at least three independent uses of the term functional, one of which is rather neutral and the other two of which have specific, sometimes conflicting, meanings. We argue that this ambiguous, undifferentiated usage can potentially contribute to diagnostic misunderstandings and treatment failures. The first of the two conflicting uses is found in expressions such as: