Open Educational Resources (OER) are teaching, learning, and research materials in any medium that permit no-cost access, use, adaptation, and/or redistribution by others. They include textbooks, curricula, syllabi, lecture notes, question banks, videos, audio, simulations, assessments, etc. 

Adapted from "Open Education: Content" (EDUCAUSE) 

Photos: By Nicole Honeywill, John Schnobrich, and Kari Shea, public domain