Adapted from the Field Guide to Consulting and Organizational Development, this pdf has concise, clear definitions of single, double, and triple-loop learning. Single loop = follow the rules; double-loop = changing the rules; triple-loop = learning about learning
Although from a religious viewpoint, the authors Ian Grayling and Kevin Commons have a concise definition of single, double, and triple-loop learning as well as a great chart of the modes of learning each one requires.
interesting website called Ventureneer by Geri Stengel. Like the paragraph: "Leading a social enterprise requires the melding of all the skills of a business owner and of a nonprofit leaders. It's another level of knowledge that teaches you how to juggle the social enterprise triple bottom line: people, planet, and profit. "
This Slideshare from ALIS on Strategic Foresight and Scenario-based Planning has a good graphic on slide 10 of single, double, and triple-loop learning.