Thing was a term that described an unknown entity, an inanimate object, or any object that was not considered to be a person.
In 2266, an android with Doctor Roger Korby's consciousness assured Christine Chapel that he did not and could not love Andrea, another android, saying "A thing is not a woman." (TOS: "What Are Little Girls Made Of?")
In the 2270s, James T. Kirk requested that Leonard McCoy to be assigned to the USS Enterprise again. When McCoy protested, Kirk began to explain, "Bones, there's a 'thing' out there..", to which McCoy retorted, "Why is any object we don't understand always called a 'thing'?" (Star Trek: The Motion Picture)