Intelligence is the associating or discriminating of things (concepts, objects etc). For example, a person who naturally associates clouds with rain is more intelligent than one who doesn't. Or also, a person who discriminates between a storm cloud and a fair weather cloud is more intelligent than one who doesn't.
Intelligence can be qualified many ways.
One way is to put it on a continuum that describes its usefulness in new (original) situations. Anything learned as part of a group forces an investment in a status quo that discourages new experience. There is a huge amount of cleverness invested in obscuring this reality. If you learn something as part of a group, you are learning to serve that group, not to serve any individual growth. There are no exceptions.
Some "intelligence" mainly involves memory.
If you are fixing a washing machine, for example, you can excel by following a set decision tree that has been followed by many others before you. You must memorize a series of steps and follow them. As you get faster at following the steps you are judged more intelligent. This kind of intelligence is learned in a group and basically describes all academic learning.
Another type of intelligence involves "starting from scratch".
The source of this is innate human nature and not any external teacher. The more this is associated with some external teacher, the less valid it is. It is never "added" to a person, only taken away. You cannot cultivate this kind of learning, except by removing those things that inhibit it. This kind of intelligence is essentially awareness.
Intelligence is a quarry for the hunter.
It has the same paradox as anything properly hunted. Specifically the more you are hunting "it" (as something you have defined), the less success the hunt will have. It's as if you see that you have 5 fingers and you say "that is good" and "I want that". How do you improve the "fiveness" of your fingers? Whatever you do, it won't end well. The same goes ultimately for everything, but in this case it goes for intelligence.