What is intelligence? Luke Muehlhauser. Attempts an explanation of intelligence.
71 Definitions of Intelligence - Shane Legg. Reviews various explanations of intelligence.
Possible Minds - John Brockman. Collection of essays about different mind architectures enabled by technology.
Society of Mind - Marvin Minsky. Classic on how intelligence on the level of the individual and society arises from individual agents cooperating with each other.
What is Intelligence? - Kurzgesagt. Intro-level explainer video defining intelligence.