The Time, Elevation, Latitude, and Longitude of what would have been the historical death of a Traveler's host body.
The first mention of T.E.L.L. occurred in the first episode when his partner, Forbes, informs pre-Traveler FBI Special Agent Grant MacLaren about a potential cell flagged due to encrypted communication in the deep web. T.E.L.L. is explicitly defined during the last moments in the episode when the group explains the mission to Grant MacLaren just before his body is taken over by a traveler.