Eating
From Memory Alpha, the free Star Trek reference
Eating, or ingestion, is one of the primary qualifiers that describe a living organism.
While observing the giant space amoeba discovered in 2268, Doctor Leonard McCoy described it as a very simple form of life capable of displaying almost all the functions living organisms perform, including the ability to eat, breathe and reproduce. McCoy, however, was not sure what it ate, but Spock speculated that perhaps it consumed energy itself. (TOS: "The Immunity Syndrome")
