Exploratory testing
Exploratory testing is simultaneous learning, test design and test execution. Since it has a highly situational structure it can seem, to the casual observer, that it has no structure at all.
It is also known as ad hoc testing. In the early 1990s, ad hoc was too often synonymous with sloppy and careless work. As a result, a group of test methodologists (now calling themselves the Context-Driven School) began using the term "exploratory" seeking to emphasize the dominant thought process involved in unscripted testing, and to begin to develop the practice into a teachable discipline. This new terminology was first published by Cem Kaner in his book Testing Computer Software. Exploratory testing can be as disciplined as any other intellectual activity.
Session-based testing is a method specifically designed to make exploratory testing auditable and measurable on a wider scale.