Time lock
A Time-Lock was a mechanism whereby an event or series of events was rendered unreachable by time travel. The Time-Lock only prevents entry into the Time War through the time vortex itself, meaning that all the events were not erased from history, and thus can still be referenced to by those who knew of the destruction of the Daleks and the Time Lords. The Doctor, however, would only be able to reach the Time War through time travel, and any entry into the events of the Time War would be considered a change in the time line (thus made impossible due to the time-lock).
Use
By Time Lords
It was known that after the Last Great Time War, the entire war was time-locked (although it was not made clear whether the lock was in place during the war itself). It was known that the Time Lords strictly forbid any change, travel, even observation into past or future events of Gallifrey. Thus, it was likely that the Time Lords put the time-lock in place immediately, for they knew that time-traveller's would constantly be changing the events of the Time War, which could be extremely dangerous to the time line. Davros suggested once that Emperors and Time Lords had attempted to breach a time-lock, but failed. Dalek Caan was able to overcome the lock on the Time War after making an emergency temporal shift, and travelled back to the Gates of Elysium to save Davros. The process flooded Caan's mind with total knowledge of the past, present and future, driving him insane. (DW: The Stolen Earth) The only other things capable of breaching the time lock was something that was already in place (like the signal that had been transmitted back through time into the Master's mind) and, on at least one occasion, very small objects (such as the White-Point Star diamond sent by Rassilon to follow the signal in the Master's head). (DW: The End of Time)
By Torchwood
Another sort of time-lock, created by Torchwood 3's resident genius Toshiko Sato, was designed to freeze the Torchwood 3 Hub in a bubble in time, which was at the same time impenetrable but inescapable. (DW: Journey's End) The time lock apparantly shut down when the Dalek trapped inside was destroyed. It was possible that that Time Lock was destroyed when the hub blew up. (TW: Children of Earth: Day One)
Other Uses
The Tenth Doctor once said in Pompeii, before it was about to be destroyed by a volcano, that certain events in the timeline were fixed points in history.' (DW: The Fires of Pompeii)
