A sterile field or steri-field was a radiation field, standard on Starfleet operating tables, which kept the surgical site completely sterile. The surgeon's hands and medical instruments were automatically sterilized each time they entered and exited the field. The field was activated manually. The steri-field replaced 20th century "scrubbing in" and sterile preps before surgery. (TOS: "Journey to Babel"; TNG: "Ethics")