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.

The Covenant lacked a direct counterpart to the human concept of sapience, though the Congruent-Enigmatic scale was used to grade the familiarity or foreignness of intelligent beings. Another means of assessing sapience was the Doctrine of Universal Conversion, which defined a species' intelligence by their ability to understand the Covenant religion and thereby their eligibility for salvation through the Great Journey.

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

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, which is another commonly-recognized trait of what is commonly understood as sapience. The capacity for tool use tends to go hand-in-hand with sapience, as the former often (albeit not always) facilitates the development of the latter. It is also 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. It has also been theorized that a biological entity may be capable of convincingly simulating the appearance of sapience (through sophisticated tool use, for example) 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.
 * Sapience

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

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 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 tool-capable 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.