Shane Legg and Marcus Hutter proposed one in 2006. The main descriptive quotes (see the paper for the actual formula):
Intelligence measures an agent’s general ability to achieve goals in a wide range of environments
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It is clear by construction that universal intelligence measures the general ability of an agent to perform well in a very wide range of environments, as required by our informal definition of intelligence given earlier. The definition places no restrictions on the internal workings of the agent; it only requires that the agent is capable of generating output and receiving input which includes a reward signal.
