Founding Titan (Anime)

The Coordinate (座標) is a mysterious item or being sought after by the warriors. It is shown that the coordinate can control mindless titans, and it is believed that it is currently held by Eren Jaeger.

Clash of the Titans arc
Before fleeing the Scouts in the Forest of Giant Trees, Reiner states that the connections between the Reiss family and the Order of the Walls make Historia Reiss a valuable asset for the warriors' mission, and having her with them will make their goal much easier if Eren Jaeger is not the "Coordinate" they are searching for.