From Halopedia, the Halo wiki
Sentience is the quality of possessing senses and the abilities of perception (eg. to perceive time).
Definitions of sentience
There are several levels of sentience:
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. Only the Precursors fit the category. The Gravemind may be transsentient, with the Forerunners sometimes viewing the Flood as a group of subsentient beings controlled by a possibly transsentient Gravemind. The Flood itself gains sentience from consuming the evolved complex neurosystem of one or more sentient entities.
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 Flood and the Halo Array
The Flood consume only organisms that are sentient or have sufficient biomass and calcium to support the infection process. Humans, most former member species of the Covenant, and many other species are sentient and thus susceptible to Flood infection. Notably, Lekgolo assemblages cannot be infected because each entity is a gestalt of numerous eel-like creatures and thus lacks a spinal cord. All muliticellular life with a neural system as simple as a notochord (including all sentient species) is vulnerable to the effect of the Halo Array.