Orbiter Vehicle Designation
Each NASA space shuttle designation is composed of a prefix and suffix separated by a dash. The prefix for operational shuttles is OV, for Orbiter Vehicle.
The suffix is composed of two parts: the series and the vehicle number.
Series:
0 - Non-flight ready shuttles
1 - Flight ready shuttles
The vehicle number is sequentially assigned within the series, beginning with 1. Therefore, there can never be an OV-100 as it would read "Orbiter Vehicle Series 1 Vehicle 0".
Many proposals to build a second generation of orbiters, externally compatible with the current system but internally new, refer to them as "OV-200" or "OV-2xx" in order to differentiate them from the "current generation", the OV-100s. This terminology is informal, and it is unlikely that any Shuttle-derived vehicle built will be given such designation.
Orbiter designations
OV-099 - Space Shuttle Challenger, originally STA-099
OV-101 - Space Shuttle Enterprise
OV-102 - Space Shuttle Columbia
OV-103 - Space Shuttle Discovery