One Artificial Intelligence definition is:
Intelligence measures an agent's ability to achieve goals in a wide range of environments. (Universal Intelligence: A Definition of Machine Intelligence by Shane Legg and Marcus Hutter, December 2007)
It means that a more intelligent agent is supposed to perform better on average. Formally, the complexity of the environment is also considered. The intelligent agent should assume that simple environments are more often. That is the Occam's razor principle.
The Strong AI tries to build the really universal intelligent machines.