March or May (5)
I believe the answer is:
month
'march or may' is the definition.
(I know that 3. month is a type of month)
This is all the clue.
(Other definitions for month that I've seen before include "One of 12 divisions" , "Four-week period" , "May, perhaps" , "Moon's period" , "Twelfth of a year" .)