backfill

Short definition from Wiktionary

Verb

1. 1
   To refill a hole with the material dug out of it.
2. 2
   To refill an excavation unit to restore the former ground surface and/or to preserve the unit and make it recognizable as having been excavated.
3. 3
   To provide reserve support.
4. 4
   To replenish or restock due to attrition or loss.
5. 5
   To enter an online game that's already in progress, replacing a player who left early.

Noun

1. 1
   The material that has been used to refill an excavation.
2. 2
   Reserve support personnel.
3. 3
   That which backfills; a replacement.
4. 4
   Material in a story set earlier in the past, providing history or context for the current action.