Time barrier
From Memory Alpha, the free Star Trek reference
The time barrier was a theoretical barrier broken in the development of warp drive in the mid-23rd century.
This obstacle was overcome sometime between the time of the crash-landing of the SS Columbia on Talos IV in 2236 and the arrival of the USS Enterprise in 2254, allowing Federation starships to travel significantly faster than ever before.
For some time after the breakthrough warp speeds were referred to as time warp factors. (TOS: "The Cage")
