Nexus-7 was the codename for a model of replicant developed by Eldon Tyrell and the Tyrell Corporation. The Nexus-7 series was believed to never have progressed beyond the prototype stage, and the only specimen known to have been produced was Rachael.
The chief difference apparent between the Nexus-7 series and previous models is that Nexus-7s were implanted with real human memories prior to their inception. While prior models would be close to their expiration date by the time that they accumulated enough memories to develop feelings and a sense of identity, Nexus-7's were thus "born" with such a wealth of memories that they could pass as human immediately, Rachael not even being aware of being a replicant.
Blade Runner Rick Deckard remarked that while previous models could be confirmed as replicants within the first 20 to 30 questions being answered during a Voight-Kampff test, Rachael answered over 100 questions before the machine could positively identify her as a replicant.
The Nexus-7 was host to an even more revolutionary technological achievement, the ability to reproduce, Rachael having borne a child by Deckard. This technology having been lost with the death of Tyrell and Rachael, Niander Wallace desperately sought to re-discover it in order to exponentially ramp up the production of replicants and thus the colonization of other planets.