Free full text Abstract Objective The term functional has a distinguished history, embodying a number of physiological concepts, but has increasingly come to mean hysterical. The DSM-V working group proposes to use functional as the official diagnostic term for medically unexplained neurological symptoms (currently known as conversion disorder). This study aimed to explore the current neurological meanings of the term and to understand its resilience. Design Mixed methods were used, first interviewing the neurologists in a large UK region and then surveying all neurologists in the UK on their use of the term. Results The interviews revealed four dominant usesnot organic, a physical disability, a brain disorder and a psychiatric problemas well as considerable ambiguity. Although there was much dissatisfaction with the term, the ambiguity was also seen as useful when engaging with patients. The survey confirmed these findings, with a majority adhering to a strict interpretation of functional to mean only not organic, but a minority employing it to mean different things in different contexts - and endorsing the view that functional would one day be a neurological construct again. Conclusions Functional embodies real divisions in neurologists' conceptualisation of unexplained symptoms and, perhaps, between those of patients and neurologists: its diversity of meanings allows it to be a common term while meaning different things to different people, or at different times, and thus conceal some of the conflict in a particularly contentious area. This flexibility may help explain the term's longevity.