What defines the definition in VCR tape? The size of the magnetic grains in the tape or something like that? Is it possible to manufacture a special magnetic VCR tape that could write DVD-quality definition?
In practice it'd just be the standard of the signal that's being recorded (PAL/NTSC/SECAM). Wouldn't surprise me if the actual tape is capable of handling more..
Consumer VHS VCRs could not reproduce the full fidelity of a broadcast NTSC picture. Commercial VCRs, which typically used Betamax, were better and used by broadcasters until digital took over.
The grains on the tape ultimately determine how high of a frequency it can record. And how close together the tracks can be.