Functional means working as it should — or made for use rather than for show. A plain chair that takes your weight.
The design sense is the interesting one, because calling a room functional is faint praise: it works, and nobody enjoyed making it beautiful. The plain sense simply means in working order, as in the lift is functional again. Medicine and psychology have their own uses, and functional illiteracy means being able to read a little but not enough to manage. The noun function is what the thing is for.
1adjectivefunctionalIn working order and able to do its job.
“Only one of the two lifts is functional.”
2adjectivedesignedDesigned for practical use rather than appearance.
“The kitchen is functional rather than pretty.”
Proto-Indo-European
*bʰewg-
Middle French
function
English
function
English, today
functional
Built from
From function + -al. function Borrowed from Middle French function. From Middle French function, from Old French fonction, from Latin functiō (“performance, execution”), from functus, perfect participle of fungor (“to perform, execute, discharge”), from Proto-Indo-European bʰewg- (“to enjoy”).
Etymology from Wiktionary, CC BY-SA