Exploratory Testing

Software Testing Glossary

Cem Kaner defines exploratory testing as "a style of software testing that emphasizes the personal freedom and responsibility of the individual tester to continually optimize the quality of his/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." on page 36 of A Tutorial in Exploratory Testing.

Also see: Exploratory Testing Dynamics by James Bach, Jonathan Bach, and Michael Bolton and What is Exploratory Testing? by James Bach.