Definitions of intelligence

Sapience is the capacity to possess wisdom, typically used synonymously with high-order intelligence. Sapience is a possible property of sentience, which is the ability to perceive and feel. Most animals are sentient, but only a fraction are sapient.

Terminology
Most living organisms are categorized as non-sentient or sub-sentient, lacking any capacity to feel, experience or perceive. These include most flora and various examples of primitive organisms.
 * Non-sentience

Some organisms possess characteristics of sentience whilst not being recognized as fully sentient. These are known to include emergent neural networks within otherwise low-order ecosystems, and various exotic entities inhabiting the energetic plasma flux of stellar nurseries.
 * Semi-sentience

Sentience is the capacity for feeling and subjective perception. While sometimes used synonymously with sapience, sentience is a much broader category; virtually all sapient beings are sentient, but only a fraction of sentient beings are sapient. Most complex biological life possessing a developed nervous system or an analogous structure is regarded as sentient.
 * Sentience

All sentient life is notably susceptible to both Flood infection and the radiation of the Halo Array, though most non-biological structures simulating sentience are unaffected.

Many sentient species occupy a vague borderline between non-sapience and sapience. Such beings are variously called as semi-sapient, pseudosapient or protosapient. Human xenobiologists have also recognized the possibility of beings referred to as cryptosapients or cryptosophonts - beings too alien to conclusively identify as possessing high-order intelligence.
 * Semi-sapience

Some terrestrial species, including higher primates, cetaceans such as whales and dolphins, and various birds such as corvids and certain species of parrot, have also been described as belonging to this category. A number of these species have been subjected to uplifting projects in the past in an effort to artificially enhance their cognitive capabilities. Though the United Earth Government effectively banned such activity in the 24th century over ethical concerns, some rogue actors are known to have since practiced it in secret. Most of the sapient species active in the Orion Arm today are also known to have (or have had) semi-sapient or protosapient evolutionary relatives, comparable to Earth's chimpanzees or orangutans. A variety of semi-sapient species have also been encountered inhabiting alien worlds, one example being the Sharquoi.

Traditionally defined as the capacity for wisdom, the very definition of sapience is notoriously broad and ever-expanding when dealing with beings different from oneself. However, it is commonly agreed to include self-awareness, symbolic and abstract thinking, introspection and reasoning. Such qualities, in turn, often give rise to culture distinct from instinctual behaviors as well as language, which are another set of commonly-recognized traits of what is commonly understood as sapience.
 * Sapience

The capacity for complex tool use and thereby technology tends to go hand-in-hand with sapience, as the former often facilitates the development of the latter. However, it is possible for species to be sapient yet physically incapable of tool use; examples include aquatic organisms with cetacean-like body plans. Others (such as natural quadrupeds lacking articulated manipulating appendages) may be tool-capable, but have a considerable threshold for developing tools due to the creative solutions required to overcome their physical limitations. In addition, technological capability may not always be an indicator of sapience. It has also been theorized that a biological entity may be capable of fairly sophisticated technology as part of an unconscious mimetic or trial-and-error mechanism without being fully self-aware, similar to the human "dumb" AIs. In such cases it may be difficult to conclusively prove such a being as being non-sapient, calling into question whether such a distinction is meaningful at all.

Sapient beings are alternatively known as sophonts, from the Greek sophós, meaning "wise".

Covenant views
Strictly speaking, the Covenant lacked a direct counterpart to the human concept of sapience, though their religious beliefs about souls or "essences" were closely related. The existence of a soul was a crucial to the Covenant, because it determined a species' eligibility to "walk the path", or embark on the Great Journey alongside the Covenant collective. This was a key part of the Doctrine of Universal Conversion, which determined a species' intelligence by their ability to comprehend the Great Journey; in order to walk to path, a species had to see the path.

According to the Covenant, a soul-essence was composed of seven aspects, roughly summarized as thus:


 * 1) Will - Possessing an independent will; an "inner fire" propelling one forward in one's world-journey
 * 2) Faith - The capacity to possess faith in the Forerunners and the Great Journey
 * 3) Reason - The ability to reason and think in abstractions
 * 4) Judgment - Having sound faculties of mind; sense, intuition and the ability to possess wisdom
 * 5) Inner Knowledge - The ability to conceptualize oneself as a distinct entity in relation to the larger frameworks of society, civilization, and the cosmos
 * 6) Contemplation - The capacity for introspection and self-improvement; of "seeing what could be"
 * 7) Communication - The ability to comprehend, possess and transfer knowledge

A being may fulfill some of these criteria, and indeed, the definitions were amended several times over Covenant history as new species were encountered, but they were not infinitely flexible. The seven-faceted soul is often visually represented as a holograph of a heptahedron, with a glyph representing each aspect on each face. This is part of a tradition of using geometric shapes to represent religious and philosophical concepts.

The Covenant also used the Congruent-Enigmatic scale to grade the nature of intelligence from the familiar to the foreign.

Frequency
To humanity, the past existence of extraterrestrial sapient life ceased to be a mere hypothetical in the 24th century, at which point various examples of out-of-place biota and a handful of ruins of technological civilizations encountered during the initial forays to other star systems made it a commonly-accepted reality. However, the important point was that all of these civilizations existed in the deep past -- until the fateful first contact with the Covenant.

Covenant scholars agree that while life in itself is relatively common in the galaxy, sentient life is rare. Sapient life is even more scarce, and out of that wide bracket, tool-using sapient life even more so; and out of tool users, only a handful achieve interstellar travel on their own. However, there are fairly reliable indications that, while sentience has always been rare, the galaxy was once far more bustling than it is today. Even the explored portions of the Milky Way are littered with the ruins of dozens of distinct cultures, often restricted to a single world albeit sometimes having spread across their home system or a small region of interstellar space. A small handful of cultures appear to have been veritable interstellar empires on the scale of the Covenant, though none found so far eclipse the extent and reach of the Forerunner civilization.

The most recent of these civilizations seem to have vanished rather recently in the galactic timeline: roughly 100,000 years ago. As this corresponds with the Forerunners' passing, the Covenant concluded that this marked the onset of their gods' Great Journey -- with the Forerunners' faithful subjects being raptured alongside their masters, while their enemies were burned away into eternal oblivion. However, the grim reality now recognized by the UNSC and increasingly by members of the Concord of Reconciliation is that as the Halo Array fired, numerous undiscovered and un-contacted cultures were snuffed out. Even some of those saved by the Conservation Measure were doomed to later die out due to archival failures and various post-reseeding cataclysms. Halo reset the galactic playing field, but only a tiny handful of once-active players reemerged. It is not known for certain how many; a Forerunner record dated in the later stages of their struggle with the Flood mentions the archival of 123 technologically-capable species, but any number of factors may offset this figure into either direction. Regardless, less than thirty sapient species are known with certainty to be active today. The frequency of non-tool users and various pseudosapients or Enigmatics is higher, albeit not massively so.

Part of this is explained by the vast scale of the galaxy; with its hundred billion stars, any starfaring culture since the Forerunners could not have hoped to explore all of it. Entire interstellar empires may exist relatively unknown to major players such as the Covenant, especially if they do not practice easily detectable large-scale megaengineering. It has been estimated that less than 0.001% of the galaxy's total number of stars have been reliably surveyed by the Covenant, most of that number being within the Orion Arm despite various expeditions to other spiral arms and even the occasional galactic circumnavigation journey.