Grey literature (or gray literature; see spelling differences) are materials and research produced by organizations outside of…

“Grey literature (or gray literature; see spelling differences) are materials and research produced by organizations outside of the traditional commercial or academic publishing and distribution channels. Common grey literature publication types include reports (annual, research, technical, project, etc.), working papers, government documents, white papers and evaluations. Organizations that produce grey literature include government departments and agencies, civil society or non-governmental organisations, academic centres and departments, and private companies and consultants.”

–Wikipedia (viainthenoosphere)