What is Behaviour Driven Development?

What is Behaviour Driven Development, according to nosewheelie?

Behaviour Driven Development (BDD) is a development practice arising out of agile development methodologies, first developed by Dan North in response to the problems experienced using and teaching TDD [10]. At its core, BDD is a refinement to TDD that shifts the emphasis from testing to specification and is in effect the best practices of what developers practising TDD have been doing all along. While this doesn’t seem like a major change, the shift in emphasis from testing to specification brings a number of important flow on effects that positively impact the development of both test and production code. The following sections describe these effects.

Heard about it from Bellware, who as a different opinion.