Eating, also known as feeding or ingestion, was one of the primary qualifiers that described a living organism. The most common form of eating of mono-form corporeal lifeforms involved ingesting organic material, which was chemically broken down by the organism's digestive system and distributed to its cells for minerals and energy. The term post-prandial was a medical term meaning "after eating". (Star Trek IV: The Voyage Home)
On the Enterprise-D group of parasitic organisms attempted to consume the crews' cellular peptides (which would have caused them to collapse into pile of chemicals and thus die). However, they were discovered and purged from their bodies. (TNG: "Phantasms")
Some lifeforms, such as the Crystalline Entity, also consumed material, but converted it to energy directly rather than chemically breaking it down. (TNG: "Silicon Avatar") Some organisms fed on energy and either required a host, such as Ronin (TNG: "Sub Rosa") or fed on it directly, such as Farpoint Station.
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")