Meaning of FUNCTIONAL in English
1》 relating to or having a function: a ~ role.
↘relating to the way in which something functions: there are ~ differences between the left and right brain.
2》 designed to be practical and useful, rather than attractive.
3》 working or operating.
4》 (of a disease) affecting the operation rather than the structure of an organ.
5》 (of a mental illness) having no discernible organic cause.
Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004