The "No-CD Patch" was a modified executable file (usually named quake3.exe ) created by the cracking scene (most notably by groups like Deviance or Razor 1911). This modified .exe bypassed the SafeDisc check, allowing the game to launch without the disc.
If you want the most stable experience on modern systems, the best "patch" isn't a crack—it's a new engine. : This is the industry-standard open-source engine Quake 3 Arena No Cd Patch
Loading assets directly from the hard drive was much faster than waiting for a 40x CD-ROM to spin up. The Official "No-CD" Fix The "No-CD Patch" was a modified executable file