"In a business as old as recorded history, one would expect to find a sophisticated understanding of just what that business is, what it does, and how it works. If the business is 'intelligence,' however, we search in vain. As historian Walter Laqueur warned us, so far no one has succeeded in crafting a theory of intelligence. I have to wonder if the difficulty in doing so resides more in the slipperiness of the tools than in the poor skills of the craftsmen or the complexity of the topic. Indeed, even today, we have no accepted definition of intelligence. The term is defined anew by each author who addresses it, and these definitions rarely refer to one another or build off what has been written before. Without a clear idea of what intelligence is, how can we develop a theory to explain how it works? If you cannot define a term of art, then you need to rethink something. In some way you are not getting to the heart of the matter. Here is an opportunity: a compelling definition of intelligence might help us to devise a theory of intelligence and increase our understanding. In the hope of advancing discussions of this topic, I have collected some of the concise definitions of intelligence that I deem to be distinguished either by their source or by their clarity. After explaining what they do and do not tell us, I shall offer up my own sacrificial definition to the tender mercies of future critics."
Center for the Study of Intelligence: https://www.cia.gov/library/center-for-the-study-of-intelligence/index.html
Studies in Intelligence (2002), v.46 no.3