replacement

noun

1. a replacing or being replaced
2. a person or thing that takes the place of another, esp. of one that has worn out, broken down, etc.
3. a member of the armed forces available for assignment to fill a vacancy or complete a quota; reinforcement
4. in crystallography, the replacing of an angle or edge by one face or more
5. Geol. the process of very gradual solution and simultaneous deposition by which one kind of mineral is substituted for another