Intelligence is a surprisingly difficult thing to define. Kurzgesagt jumps into the debate with an interesting overview of where intelligence begins. Is a slime mold intelligent? Are plants intelligent?
They don't really go too deep into human intelligence, but it's something I know a lot about it from my tour of duty in the Wikipedia wars on articles covering intelligence. The most interesting work to me was that of Howard Gardner, who proposed that humans have multiple intelligences. Most intelligence tests only measure a couple of them. He proposed eight kinds that meet his criteria for what constitutes human intelligence:
Image: YouTube / Kurzgesagt