substitute

noun

1.
a person or thing that takes or can take the place of another
back-up standin swapfiller replacement
Derived from:
substitute substitute
2.
an athlete who plays only when a starter on the team is replaced
reserve second-stringer
Derived from:
substitute substitute
3.
someone who takes the place of another (as when things get dangerous or difficult)
backup fill-in relief reliever stand-in
Derived from:
substitute substitute