R. J. Sternberg quoted in a paper by Shane Legg: “Viewed narrowly, there seem to be almost as many definitions of intelligence as there were experts asked to define it.”

Did you read the conclusion of that paper, where Legg and Hutter mentioned another of their papers that proposes a formal definition of intelligence? Their definition isn't perfect and it is easy to sneak in connotations, but that is far better than a definition that makes no predictions.