What is the term for the ability of an organism to keep things the same?

If you are referring to the process of maintaining the processes within the organism essential for life constantly going, it is called "maintaining homeostasis." so when someone asks, "Watcha doin'?" you can respond, "Maintaining homeostasis."

Pumping blood, converting sugar to energy, replicating cells, etc. are all examples of the countless things required in order to maintain homeostasis.