reserve
noun
1.
formality and propriety of manner
2.
3.
an athlete who plays only when a starter on the team is replaced
4.
(medicine) potential capacity to respond in order to maintain vital functions
5.
a district that is reserved for particular purpose
6.
armed forces that are not on active duty but can be called in an emergency
Derived from:
reservist 7.
the trait of being uncommunicative; not volunteering anything more than necessary