A set of principles for developing products in the face of uncertainty, as described by Eric Ries in the book 

The Lean Startup --- http://ecx.images-amazon.com/images/I/51vN15YCJyL.jpg
* Audio Sample
*** http://samples.audible.com/bk/rand/002759/bk_rand_002759_sample.mp3
EricRies
* related blogs and articles
*** http://theleanstartup.com/
* Principles
*** http://theleanstartup.com/principles

A key aspect is using experiments to quickly validate our hypotheses about users/customers.  

See also 
* SpikeProblem -- KelleyHarris