Constancy
Constant, Unchanging
noun
Keeping things constant and normal ensures constancy.
恒常 is a formal word used to describe something that remains constant and unchanging.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
このシステムで部屋の温度を恒常に保っています。
With this system the room is kept at a constant temperature.