in the TMP novelization, shields referred to the standard TOS shields - several separate forcefield defenses that might overlap, but still had gaps. the forcefield was mentioned as a new system - one single bubble that had no gaps. It seemed to be more powerful than any individual shield, but provide less overall protection than the shields. It's important contribution was that an enveloping attack like V'Ger's weapon could get around shields, but not the forcefield. Seeing a single bubble effect in TMP led me to figure that over time, Starfleet merged the two related systems into one, and so sometime after TMP the older term was retained for the new unified defense, which replaced the older, segmented defense.