The double diamond diagram was developed through in-house research at the Design Council in 2005 as a simple graphical way of describing the design process.
Divided into four distinct phases, Discover, Define, Develop and Deliver, it maps the divergent and convergent stages of the design process, showing the different modes of thinking that designers use.
http://www.designcouncil.org.uk/en/About-Design/managingdesign/The-Study-of-the-Design-Process/