The ability to form and make use of a well-corresponding and predictive model of one's experiential environment / situation (at whatever scope you like) in order to realize goals, or reach conclusions that could be shown to be 'true' in the environment.
One may speak of the practically realized ability, or alternately the capacity to develop the ability.
Human Intelligence = Ability to navigate environments in conceptual space. An evolution from navigation in physical space.
Think about how when you start learning a new concept, you're like a person waking up in a new environment they've never seen before.
You look around for points of reference and start to build up a map of the environment.
Notice how we use the same words for understanding a concept as we use for physical navigation.
If you've ever tried the mind palace memorization technique, you may have wondered why things are easier to remember when you link them to a physical space, this is why.
There are so many. One of my favorites, a bit of a unification theory: Intelligence can be described as the degree of alignment (of a person, statement, idea, test, etc.) to intuitive-logical type psychology, known today as the Theorist Temperament among other titles.
We can divide this temperament into four sets of types, among which the two introverted types seem (anecdotally at least) to be overrepresented in text-based technical-theoretical communications.
We can then back up a bit and look at introversion through the model of "dopamine sensitivity" and this overlays nicely with the "drab" or "organized but ordinary" appearance of technical-theoretical websites, documents, etc.
I've discussed this with Linda Berens (below) and there are other ramifications which are really, really fascinating. She has done a lot of work to modernize and unify ages-old theories on personality, and it is intriguing how readily intelligence concepts are easily subsumed into such theories.
Most impressive to me, the various human preferences are laid out on a more even playing field than is allowed in some corners of human psychology, which is probably good. For one, you can say that this way of looking at things leaves more room for i.e. "emotional intelligence," "relational intelligence," "memory as intellect," and definitely to include the other commenter's book smart, street smart, hustle, etc.
In this way you can start with a concept like "OK, certain types of people REALLY value the concept of intelligence, and here's how they think," but then you also end up looking over at their blind spots, with the ability to define those blind spots as a liability, even in the face of all that intellect. Which is a nice advantage as it helps refine the general concept of intellect, and also informs a sort of advocacy for other important things to have, above and beyond just intellect.
One may speak of the practically realized ability, or alternately the capacity to develop the ability.