John McCarthy long ago gave one of the best definitions:
"Intelligence is the computational part of the ability to achieve goals
in the world”. That is pretty straightforward and does not require a
lot of explanation. It also allows for intelligence to be a matter of
degree, and for intelligence to be of several varieties, which is as it
should be. Thus a person, a thermostat, a chess-playing program, and a
corporation all achieve goals to various degrees and in various senses.
For those looking for some ultimate ‘true intelligence’, the lack of an
absolute, binary definition is disappointing, but that is also as it
The part that might benefit from explanation is what it means to achieve goals. What does it mean to have a goal? How can I tell if a system really has a goal rather than seems to? These questions seem deep and confusing until you realize that a system having a goal or not, despite the language, is not really a property of the system itself. It is in the relationship between the system and an observer. (In Dennett's words, it is a ‘stance’ that the observer take with respect to the system.)
What is it in the relationship between the system and the observer that makes it a goal-seeking system? It is that the system is most usefully understood (predicted, controlled) in terms of its outcomes rather than its mechanisms. Thus, for a home-owner a thermostat is most usefully understood in terms of its keeping the temperature constant, as achieving that outcome, as having that goal. But if i am an engineer designing a thermostat, or a repairman fixing one, then i need to understand it at a mechanistic level—and thus it does not have a goal. The thermostat does or does not have a goal depending of the observer. Another example is the person playing the chess computer. If I am a naive person, and a weaker player, I can best understand the computer as having the goal of beating me, of checkmating my king. But if I wrote the chess program (and it does not look very deep) I have a mechanistic way of understanding it that may be more useful for predicting and controlling it (and beating it).
Putting these two together, we can define intelligence concisely (though without much hope of being genuinely understood without further explanation):