That's an unofficial designation.

OBD1 and OBD2 are actual systems mandated by the US DOT.

Honda changed their ECUs over to a different style in the '01 Civic, RSX, etc. These still conform to the OBD2 spec but because they are different from the first generation of Honda OBD2 ECUs they're sometimes called OBD2.5 just for the sake of clarification.