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.
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)
On one planet, the Second Doctor described an object which appeared to be a strange cloud as "an echo of some cosmic events across the corridors of Time." The army of humanoids entering the cloud were unable to see or hear the Doctor, and passed through him as if they were ghosts. Jamie McCrimmon followed them through the cloud and was almost made their leader in their battle against the Scythias. (PROSE: Follow the Phantoms)