To quote Wikipedia:
"Intelligence has been defined in many different ways such as in terms of one's capacity for logic, abstract thought, understanding, self-awareness, communication, learning, emotional knowledge, memory, planning, creativity and problem solving. It can also be more generally described as the ability to perceive information and retain it as knowledge for applying to itself or other instances of knowledge or information, thereby creating referable understanding models of any size, density, or complexity, due to any conscious or subconscious imposed will or instruction to do so."
Therefore, intelligence is mostly subjective towards the circumstance.
Despite variables, some suggested intelligence levels are as follows:
- Non-Sentient: Self-explanatory.
- Learning Impaired: Candidate can only comprehend one concept, and is incapable of processing more than one at a time.
- Below Average: Characters that show lower cognitive ability than the norm, but do not particularly stand out in any intellectual or academic fields.
- Average: Regular I.Q. and no special abilities or displays, nor cognitive ability.
- Above Average: Characters that show greater cognitive ability than the norm, but do not particularly stand out in any intellectual or academic fields.
- Gifted: Character that demonstrate high reasoning ability, can master concepts with few repetitions, and display high performance capability in intellectual, creative, or specific academic fields.
- Genius: Genius level intelligence in one, or a few, areas of research.
- Extraordinary Genius: Far above real world human intelligence in multiple fields of research. Capable of inventing futuristic technology.
- Supergenius: Vastly superhuman intelligence in virtually all fields of research.
- Nigh-Omniscient: Candidate almost knows absolutely everything within an area, with the exception of a few things here and there.
- Omniscient: Candidate knows anything, everything and everyone within an area.
There should always be explanations for the ratings within a following parenthesis, unless it's hard to describe or nearly impossible.