There are many definitions, but one that is widely quoted is from Shulman,
"The scholarship of teaching and learning invites faculty...to view teaching as serious, intellectual work, ask good questions about their students' learning, seek evidence in their classrooms that can be used to improve practice, and make this work public so that others can critique it, build on it, and contribute to the wider teaching commons."
Hatch, Thomas, Melissa Eiler White, and Lee S. Shulman. Into the Classroom: Developing the Scholarship of Teaching and Learning. San Francisco: Jossey-Bass, 2006.