Trio or triad (5)
I believe the answer is:
three
I believe this is a double definition.
'trio' is the first definition.
(I know that trio can be written as three)
'triad' is the second definition.
(I know that triad can be written as three)
(Other definitions for three that I've seen before include "Number of feet in a yard" , "III" , "Number of strikes before you're out" , "Prime number" , "Amount in a trio" .)