One of the most definitive things ever said regarding the nature of intelligence was that intelligence is whatever IQ tests measure.
The actual score that a person gets is dependant on how others in that age group do on those particular questions.
While the IQ test is generally used and widely accepted they have been criticized for several different reasons.
Of course by such a definition even a person who is generally considered to be dull can be regarded as being intelligent if he can take care of himself.
Other definition is such as having the tendency to analyze things around yourself.