A temporal echo was a space-time event in which those experiencing the echo could not be seen nor could they touch anything until someone outside the echo touched the object. The Doctor and his companion Guinevere Winchester experienced a temporal echo on the Schrödinger Institute. (PROSE: Revenants) When Ace saw another police box while the TARDIS was trapped in a temporal plexus, the Seventh Doctor called it a temporal echo. Because time and space were so mixed up where they were, she could've seen them ten minutes ago or a decade in the future. (PROSE: The Ripple Effect)