Popularly, a nonvariable. In theological terms that which continues to exist while it develops. The apparent contradiction between that which remains constant while also growing is one of the phenomena of Christianity. Thus the Church remains essentially the same even as she changes in adapting to different cultures and times. And revelation stays basically what it was at the end of the apostolic age, while showing remarkable growth in depth and intelligibility over the centuries. What changes is the human, what remains constant is divine.