The most common definition of grey literature, the so-called 'Luxembourg definition', was discussed and approved during the 3rd International Conference on Grey Literature in 1997. In 2004, at the 6th International Conference on Grey literature in New York City, a postscript was added. The main characteristic of this definition is its economic perspective on grey literature, based on business, publishing and distribution models of the disappearing Gutenberg galaxy. With the changing research environment and new channels of scientific communication, it becomes clear that grey literature needs a new conceptual framework. Research method: Our project applies a two-step-methodology: (1) A state of the art of terminology and definitions of the last two decades, based on contributions to the GL conference series (1993-2008) and on original articles published in The Grey Journal (2005-2010). (2) An exploratory survey with a sample of scientists, publishing and LIS professionals to assess attitudes towards of the New York definition and to gather elements for a new definition. Results: Based on the state of the art and the survey data, we make a proposal for a new definition of grey literature ("Prague definition") with four new essential attributes: "Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by library holdings or institutional repositories, but not controlled by commercial publishers i.e., where publishing is not the primary activity of the producing body." The attributes and challenges are discussed.