I was thinking about this earlier, and I think it can mean: having very good reasoning abilities (e.g. being able to give suitable answers to 'why' questions), making links between things and understanding new concepts relatively quickly.
I think that intelligent people have a greater capacity to learn and retain knowledge, so it's not 'no link', but just an element of intelligence. They're also better at learning important evaluation skills that help them process new information successfully, or at the very least to appreciate the dimensions of a problem, even if they don't immediately understand them.
I like to think of it as everyone could be placed out of ten - half of the points available for mental aptitude that you're born with, half for skills and knowledge that you acquire. A 'six' could be five of one, one of the other. Only the 'five plus five' people are the real geniuses, so it's a good job we don't need too many of those .
I do know a brilliantly well-educated guy who places enormous importance on intellectualism, and is incredibly good at repeating arguments, but he can't debate with you. It's all a regurgitation of other people's ideas, and he can't easily deconstruct ones presented to him on the spot.
I expect people will be along in a minute to say that everyone is intelligent in their own way and that there are different kinds of intelligence (that I don't deny - it's way more complex than I'm saying here, obviously). Most people like to think of themselves of intelligent though, and any notion that it's in any way pre-determined is a fact that some people don't like to face.
Being mentally quick and perceptive and making connections between concepts easily and with good understanding of how and why those connections are.
This then helps you to learn things, retain information and recall it to be used in the correct context (I think having lightning wit is a marker of intelligence) but I don't think just knowing loads of stuff is a sign of intelligence in itself.
To be able to think about information in either a logical or creative way and be able to have a critical stance on some things, e.g. not just recite the opinions or behaviours of others.
For example, My graphic designer DP who failed most exams in his life, is someone I would deemed highly intelligent. A different intelligence from me doing a masters of science, but intelligent all the same!
Yy to understanding and learning things easily and quickly. Being able to explain or communicate things well isn't necessarily a sign of high intelligence. There are definitely very clever people who can't explain for toffee!
As for the knowledge thing - a person who is intelligent doesn't necessarily have to be knowledgeable, or vice versa, but the two often go together because intelligent people usually find it easier to attain (and retain) knowledge. Not sure if having a good memory is necessary in order to qualify as intelligent, but it definitely helps with the knowledge side of things.
I think people think of the speed with which someone acquires new skills - but actually that's nowhere near as important as people in the UK think. Intelligence as quickness is a very culturally embedded concept that isn't that relevant out there in the wider world, where intelligence as depth is more common.
Good question. I have a very good ability to learn and retain new knowledge very quickly, however, I struggle to apply this to 'actual' real life situations. Do I consider myself intelligent? Yes. But there are areas which I fall down. I suppose everyone has their own way of processing things. I can explain complex philosophical concepts very well but I can't change a tire even though in theory I can explain to you (or someone) how it's done having read how to do it.