The Hyperdyne Systems 120-A/2 was a humanoid Synthetic manufactured by Hyperdyne Systems during the 22nd century. One of these models, known as Ash, was placed aboard the USCSS Nostromo as a sleeper agent in 2122.
While incredibly advanced for their time (able to successfully mimic human behavior to the point of posing as humans themselves), the 120-A/2 was widely regarded as unreliable, volatile and even dangerous in later years. Their tendency to malfunction led to the introduction of behavioral inhibitors in Synthetics, preventing subsequent models from harming, or allowing harm to come to, humans.
Behind the Scenes
Originally, the Hyperdyne Systems 120-A/2 was going to be the "Cyberdyne Systems 120-A/2", which would have been a much more direct reference to Cyberdyne Systems (aka Cyberdyne Systems Corporation) from the Terminator franchise.