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