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

A sentient being is a being with the ability to think intelligently, commonly determined by the being's ability to speak, manipulate tools, and reason. Other lifeforms lacking such capabilities are classified as non-sentient, though some classification systems have a middle ground, semi-sentient, for species that had very primitive capabilities to manipulate tools, use language, and/or solve problems. The Precursors were thought to be "transsentient", which probably means that they were above "normal" sentient life forms in factors such as intelligence.

In Relation to the Halo Array and the Flood
The Flood only consumed an organism that was sentient and also had a sufficient biomass along with calcium to support the infection process. Thus, they are not considered "sentient", as the Halo was designed to kill the Covenant, Humans, and their food. For the most part, all species in the Covenant and Humanity are sentient, but some lack the structural integrity needed to be infected; for example, Hunters cannot be infected because each entity is made from a buildup of many eels that lack a spinal cord to control. It is, as of now, unknown how much mass is needed to be of sufficient use for Flood infection.