The material that has been used to refill an excavation.

Reserve support personnel.

That which backfills; a replacement.

Material in a story set earlier in the past, providing history or context for the current action.

To refill a hole with the material dug out of it.

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.

To provide reserve support.

To replenish or restock due to attrition or loss.

"The company backfilled Joe's position after he was terminated."