There are actually a few things that, although seperate, come together in what we might typically think of as intelligence.
The ability to simultaneously consider a large number of knowns and unknowns, and then proceed to make useful (situationally accurate) inferences.
The ability to develop and retain methods for manipulating knowledge to produce derived knowledge.
The ability to establish conceptual boundaries (limits), thus allowing one to classify without (completely) defining.
The ability to accept inference as provisionally sound.
The ability to consider causality in both directions.