Ancient:

An Ancient is a team’s main base structure in Dota 2, found at the center of each side’s base. If a team destroys the enemy Ancient, it wins the match.

The Ancient is the final objective in a game, so pushing toward it usually means breaking through enemy defenses and clearing the base first. The word can also refer to the Ancient camp in the jungle, but in normal gameplay and strategy discussions it most often means the base building.