Intelligence is not only the ability to reason; it is also the ability to find relevant material in memory and to deploy attention when needed.

Intelligence is what you use when you don't know what to do.

Intelligence is quickness to apprehend as distinct from ability, which is capacity to act wisely on the thing apprehended.

Intelligence is traditionally viewed as the ability to think and learn. Yet in a turbulent world, there’s another set of cognitive skills that might matter more: the ability to rethink and unlearn.

Intelligence is not the ability to store information, but to know where to find it.

Intelligence, then, is the ability to attain goals in the face of obstacles by means of decisions based on rational (truth-obeying) rules. The computer scientists Allen Newell and Herbert Simon fleshed this idea out further by noting that intelligence consists of specifying a goal, assessing the current situation to see how it differs from the goal, and applying a set of operations that reduce the difference. Perhaps reassuringly, by this definition human beings, not just aliens, are intelligent. We have desires, and we pursue them using beliefs, which, when all goes well, are at least approximately or probabilistically true.

Knowing a great deal is not the same as being smart; intelligence is not information alone but also judgement, the manner in which information is co-ordinated and used. Still, the amount of information to which we have accessed is one index of our intelligence.

Intelligence is the ability to adapt to change

Intelligence is the capacity to perceive the essential, the what is; and to awaken this capacity, in oneself and in others, is education.

No one knows where the borderline between non-intelligent behavior and intelligent
behavior lies; in fact, to suggest that a sharp borderline exists is probably silly. But
essential abilities for intelligence are certainly:
to respond to situations very flexibly;
to take advantage of fortuitous circumstances;
to make sense out of ambiguous or contradictory messages;
to recognize the relative importance of different elements of a
situation;
to find similarities between situations despite differences which may separate them;
to draw distinctions between situations despite similarities may link them;
to synthesize new concepts by taking old them together in new ways; to come up
with ideas which are novel.

Historically, people have been naive about what qualities, if mechanized, would undeniably constitute intelligence. Is intelligence an ability to integrate functions symbolically? If so, then AI already exists, since symbolic integration routines outdo the best people in most cases.
If intelligence involves learning, creativity, emotional responses, a sense of beauty, a sense of self, then there is a long road ahead, and it may be that these will only be realized when we have totally duplicated a living brain.

Intelligence isn't what you know; it's how many of your own problems you can solve.

Intelligence is not merely the capacity which enables us to profit by experience; it is the function of personality which gives experience its past, present and future meaning. Habits belong to existence, intelligence to living. Life becomes a creative venture in proportion to the amount and quality of intelligence which accompanies conduct.

... intelligence is not just a matter of acting or behaving intelligently. Behavior is a manifestation of intelligence, but not the central characteristic or primary definition of being intelligent. A moment's reflection proves this: You can be intelligent just lying in the dark, thinking and understanding. Ignoring what goes on in your head and focusing instead on behavior has been a large impediment to understanding intelligence and building intelligent machines