When one chooses (2,4)
I believe the answer is:
at will
'when one chooses' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for at will that I've seen before include "When I choose" , "Whenever one pleases, say fire rifle" , "According to one's own inclination" , "Fire .. .... - whenever you're ready" , "Freely" .)