Online education, both by for-profit institutions and within traditional universities, has seen recent tremendous growth and appeal - but online education has many aspects that are not well understood. The SAGE Encyclopedia of Online Education provides a thorough and engaging reference on all aspects of this field, from the theoretical dimensions of teaching online to the technological aspects of implementing online courses–with a central focus on the effective education of students.This entry focuses on Distance Education: Distance education (DE), also known as distance learning, has gone through a multitude of iterations, and theorists disagree on its defining features.