func·tion·al / ˈfəngkshənl/ • adj. 1. of or having a special activity, purpose, or task; relating to the way in which something works or operates: there are important functional differences between left and right brain. ∎ designed to be practical and useful, rather than attractive: she had assumed the apartment would be functional and simple. ∎ working or operating: the museum will be fully functional from the opening of the festival. ∎ (of a disease) affecting the operation, rather than the structure, of an organ: functional diarrhea. ∎ (of a mental illness) having no discernible organic cause: functional psychosis.2. Math. of or relating to a variable quantity whose value depends on one or more other variables.DERIVATIVES: func·tion·al·ly adv. functionally, the role of the library service is clearly educational.