What is the name of Eleven's biological mother in "Stranger Things"?

Eleven's biological mother in "Stranger Things" is Terry Ives.

Terry Ives was a young college student who participated in experiments connected to the CIA's MKUltra program in the late 1960s. Without realizing she was pregnant, she was exposed to drugs, sensory-deprivation procedures, and other experiments overseen by Dr. Martin Brenner. Her daughter, Jane, was taken from her at birth and brought to Hawkins National Laboratory.

Brenner concealed the abduction by making Terry and others believe that she had suffered a miscarriage. Terry never accepted that explanation. She eventually broke into Hawkins Lab and found Jane, but lab workers captured her and subjected her to electroshock treatment, leaving her with severe brain damage and only fragmented memories.

Eleven later reconnects with Terry through those memories and learns more about her origins. Terry is sometimes confused with Becky Ives, who is Eleven's aunt, or Joyce Byers, who becomes an important parental figure. Neither is her biological mother. Terry's full name is Teresa Ives, though the series commonly uses the shorter form Terry.