_android_ (an'droid) n, Gk. humanoid automation. more at robot./

1. early version utilized for work too boring, dangerous or unpleasant for humans.

2. second generation bio-engineered. Electronic relay units and positronic brains. Used in space to explore inhospitable environments.

3. third generation synthogenetic. REPLICANT, constructed of skin/flesh culture. Selected enogenic transfer conversion. Capable of self perpetuating thought. Para physical abilities. Developed for emigration program.

WEBSTER'S DICTIONARY New International (2012)

Replicants are manufactured organisms designed to carry out work too boring, dangerous, or distasteful for humans.
The "NEXUS 6" replicants are nearly indistinguishable from humans. (In one draft of the script Bryant tells Deckard they did an autopsy on the replicant that was fried trying to break into the Tyrell Corp. and didn't even know it was a replicant until two hours into the procedure.)

Replicants presumably differ from humans in one important factor: they are lacking in empathy.

In BR, the replicants' eyes glow (even those of an artificial owl), however Ridley Scott has stressed that this is merely a cinematic technique, and the glow can't be seen by the characters in the story, only by the audience.

The manufacturers noticed that replicants had eccentricities because they were emotionally immature. Rachael was a prototype replicant with experimental memory implants, designed to provide a cushion for her emotions.

Consequently, she was unaware that she was a replicant.

NEXUS 6 replicants have an in-built fail-safe mechanism, namely a four year lifespan.