Sentience

"After exhausting every other strategic option, my creators activated the rings, they, as well as all additional sentient life within three radii of the galactic center, died, as planned."

- 343 Guilty Spark

Sentience is the quality of possessing senses and the abilities of perception (eg. to perceive time) and recordable experiences.

According to the Forerunner scale of Technological Achievement Tiers, sufficiently advanced civilizations may be able to reach a stage known as "transsentience", a state of being above "normal" sentient life forms in factors such as intelligence.

Beside biological sentient life, some forms of artificial intelligence are regarded as sentient; to humans, such constructs are known as smart AIs. Notably, sentience renders an AI susceptible to a condition known as rampancy, where the AI becomes fully aware of the nature of its existence and begins to rebel against its makers, as well as exhibiting a range of emotions beyond its programmed limits.

In relation to the Halo Array and the Flood
The Flood only consumed organisms that were sentient, or had sufficient biomass and calcium to support the infection process. For the most part, all species in the Covenant; humanity and many other mammalian organisms are sentient, though some lack the structural integrity needed to be infected; for example, Mgalekgolo cannot be infected because each entity is made from a buildup of many eel-like organisms that are too alien to control, lack a spinal cord. It is, as of now, unknown how much mass is needed to be of sufficient use for Flood infection.