Full Idea
The function of anything is what it alone can do, or what it can do better than anything else.
Clarification
'Function' is the Greek word 'ergon'
Gist of Idea
A thing's function is what it alone can do, or what it does better than other things
Source
Plato (The Republic [c.371 BCE], 353a)
Book Reference
Plato: 'Republic', ed/tr. Waterfield,Robin [OUP 1993], p.40
A Reaction
I take this concept to be the lynchpin of Aristotle's virtue ethics. Note that it arises earlier, in Plato. Perhaps he should say what it is 'meant to do'.