Agility

Functional Specification
"Typically, the functional specification for an application program with a series of interactive windows and dialogs with a user would show the visual appearance of the user interface and describe each of the possible user input actions and the program response actions." "At least one major product development group used a 'Write the manual first' approach. Before the product existed, they wrote the user's guide for a word processing system, then declared that the user's guide was the functional specification. The developers were challenged to create a product that matched what the user's guide described."
 * http://searchsoftwarequality.techtarget.com/sDefinition/0,,sid92_gci212169,00.html

"the functional specification is really a collection of documents (including the use cases) that define the functional requirements of the system"
 * http://www.softwarereality.com/lifecycle/functionalspec.jsp