I kind of agree there is no hard definition because what's "complex" and "reduced" are subjective things. The venerable 6502 was called "RISC before it was cool" and I have to agree its minimalistic ISA made it incredibly capable (and fast, clock for clock) compared to its more ambitious contemporaries such as the Intel 8080/8085, the Motorola 6809, and its arch-nemesis, the Z-80.
The 6502 is nothing like what we would call RISC today (the 6800 is much more modern RISC-like), but the evolution from 6800 to 6502, by largely the same design team at a new company, does somehow follow the RISC "quantitative approach" of getting the most performance from a given number of transistors (the 6800 and 6502 are about the same size), but it does it by adding complexity rather than by simplifying.
> but it does it by adding complexity rather than by simplifying.
That's true - it simplifies a lot of other things alongside the ISA, but it's also apparent that, while it can do a lot of tasks faster than, say, a Z-80, the ISA forces one to take some creative approaches.
Literal ports of Z-80 code to the 6502 usually sucked.