My definition of Exploratory Testing

Exploratory testing involves a tester using their knowledge and experience to explore a website or app, learn it’s layout, contents and functions whilst simultaneously designing and executing test scenarios and test cases. This unscripted approach can lead to issues being found almost immediately.

Wikipedia definition of Exploratory Testing

Exploratory testing is an approach to software testing that is concisely described as simultaneous learning, test design and test execution.
http://en.wikipedia.org/wiki/Exploratory_testing

Cem Kaner’s (and others) definition of Exploratory Testing

“Exploratory software testing is a style of software testing that emphasizes the personal freedom and responsibility of the individual tester to continually optimize the value of her work by treating test-related learning, test design, test execution, and test result interpretation as mutually supportive activities that run in parallel throughout the project.”
http://kaner.com/?p=46

Michael Bolton’s definition of Exploratory Testing

“simultaneous (or parallel) test design, test execution and learning”
http://kaner.com/?p=46#comment-26
 

Video

Exploratory Testing playlist on YouTube