Real Dictionary

function key
countable noun: Function keys are the keys along the top of a computer keyboard, usually numbered from F1 to F12. Each key is designed to make a particular thing happen when you press it.

function key in American English
noun: a key on a computer terminal or microcomputer keyboard that, when pressed, causes a specific computational or mechanical operation to be carried out

function key in British English
noun: a key on the keyboard of a microcomputer, etc. that gives special commands to the computer