A Replicant is a synthetic, biorobotic being with para-physical capabilities and designed to resemble a living, organic being. It is a genetically engineered being composed entirely of organic substance.
Animal replicants (animoids) were developed first for use as pets and beasts of burden after most real animals became extinct. Later, humanoid replicants were created for military purposes and for the exploration and colonization of space. The Tyrell Corporation introduced the Nexus-6, the supreme replicant — much stronger and faster than, and virtually indistinguishable from real human beings. Earth law forbids replicants on the planet, except in the huge industrial complex where they are created. The law does not consider replicants human and therefore accords them no rights nor protection.
A key aspect of replicant psychology is that they are lacking in empathy, in effect making them textbook sociopaths. Because of that, measuring the degree of empathic response via a Voight-Kampff machine is the most used method of detecting replicants. Nexus-6 replicants also have an in-built fail-safe mechanism, namely a four-year lifespan, to prevent them from developing empathic cognition and therefore immunity to a Voight-Kampff machine. This is especially necessary for Mental-A models whose intellectual capacity at least matches their designers. Due to their short lifespans, replicants had no framework within which to deal with their emotions, which lead to them being emotionally inexperienced. Tyrell sought to change this by gifting replicant with a past through implanted memories and therefore creating an emotional cushion that would make them far more controllable. This vision lead to the creation of Rachael, a replicant who was implanted with the memories of Tyrell's niece and believed herself to be human.
Replicants are illegal on Earth after a bloody mutiny against an off-world colony staged by a Nexus-6 combat model. Special police units (Blade Runners) are sent to investigate, test and ultimately "retire" (kill) replicants found on Earth.
The debate concerning Deckard's replicant status continues despite director Scott's repeated clarifications.