a.

1. 机能的
2. 职务上的；在起作用的
3. 实用的
4. 【数】函数的

adj.

1. relating to or having a function:
   a functional role.
   ▸relating to the way in which something functions:

   there are functional 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.