First generation cross, F1.
Offspring of first generation cross, F2.
Offspring of F2, F3. Etc.
I would still consider the offspring to be HBG, although popular concensus states that the further removed from the original F1's the offspring are, the less desireable they become due to loss of hybrid vigor.
This is where the oft repeated warning about HBG turning into GSF, comes from. Scientifically impossible, but for practical intents and purposes, a possible concern. I still recommend always stocking HBG with a predator capable of severely curtailing recruitment, but my experiences thus far seem to point to the F2 and F3 generations as showing good potential for growth also. But, my HBG endeavors are not typical of most pond settings.
