No-time, or non-time, was a form of time that existed outside the normal universe.
The Time Vortex was composed of no-time and no-space, (AUDIO: The Forbidden Time) just as interstitial time was an envelope of non-time and non-space underlying the "real" universe, surrounding and separating every spacetime event. (PROSE: Falls the Shadow)
Class B3 of Coal Hill Academy was transported to no-time, no-space when an asteroid entered a tear in space-time in the hallway. While there, one could not age and did not have to eat to survive. (TV: Detained)
When two incarnations of the Master switched their minds and bodies, thereby creating a paradox were one couldn't exist because of the change, the resulting paradox caused gaps in history, areas of no-time wherever the two Masters went, leaving only islands of time. When the universe was eventually destroyed, the Seventh Doctor utilised the anomaly cage to "reboot" the universe, with only a few minor changes, returning the Masters to their correct places in time and space, and bringing a young woman back to life. (AUDIO: The Two Masters)
During the Last Great Time War, the Technomancers had access to non-time in a crypt in Aldriss to pluck creatures that "never existed" to sacrifice to the Horned Ones. (AUDIO: Legion of the Lost)
When the Fifth Doctor regenerated, he experienced no-time, suspended in a kind of limbo. (PROSE: The Caves of Androzani)