Extelligence is a term coined by Ian Stewart and Jack Cohen in their 1997 book Figments of Reality.
They define it as the cultural capital that is available to us in the form of external media (e.g. tribal legends, folklore, nursery rhymes, books, videotapes, CD-ROMs, etc.)
'Complicity' is a combination of complexity and simplicity, and Cohen and Stewart use it to express the interdependent relationship between knowledge-inside-one's-head and knowledge-outside-one's-head that can be readily accessed.
Although Cohen's and Stewart's respective disciplines are biology and mathematics, their description of the complicity of intelligence and extelligence is in the tradition of Jean Piaget, Belinda Dewar and David A. Kolb.
Cohen and Stewart propose the idea that each individual can access the parts of the extelligence space with which their intelligence is complicit.