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 ability to think intelligently, commonly determined by the being's ability to speak, manipulate tools, and reason. 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.

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.

In relation to the Halo Array and the Flood
The Flood only consumed organisms that were sentient and had sufficient biomass and calcium to support the infection process. 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.

List of known sentient species in Halo

 * Forerunner
 * Human
 * Huragok
 * Jiralhanae
 * Kig-yar
 * Lekgolo
 * Precursor
 * Sangheili
 * San 'Shyuum
 * Unggoy
 * Yanme'e