a key etc that you keep in addition to the one you usually use, so that it is available if the one you usually use breaks, gets lost etc:
a spare key
Bring a towel and some spare clothes.
a supply of spare batteries
a spare tyre
not being used or not needed at the present time:
not used/needed[usually before noun]
Have you got any spare boxes?
You could sleep in the spare bedroom.
Do you have any spare cash.
I'll go and see if there are any spare seats.
A decline in beer sales had left the industry with spare capacity (=the ability to produce more than can be sold).
time when you are not working:
What do you do in your spare time?
Eric spent every spare moment he had in the library.
coins of little value that you do not need and can give to other people:
There are beggars on every corner asking for spare change.
5 British English spoken
if something is going spare, it is available for you to have or use:
I'll have some of that cake if it's going spare.
6 British English informal
to become very angry or worried:
Dad would go spare if he found out.
a spare style of writing, painting etc is plain or basic and uses nothing unnecessary
someone who is spare is tall and thin