A Massive Open Online Course is an online course aimed at unlimited participation and open access via the web. In addition to traditional course materials such as filmed lectures, readings, and problem sets, many MOOCs provide interactive user forums to support community interactions between students, professors, and teaching assistants. MOOCs are a recent development in distance education.
Source: Wikipedia

Massive Open Online Course (MOOC) is an online course available for large enrolment on the open web, where open largely refers to open registration, and not necessarily courses in open licence.
Source: COL 2015, Global

Massive Open Online Courses (MOOCs) are online courses designed for a large number of participants that can be accessed by anyone anywhere, as long as they have an internet connection.
Two forms of MOOCs have emerged. In cMOOCs, learners are encouraged (though not required) to contribute actively via blog posts, tweets or other social media posts that are aggregated online by course organisers and shared with all participants via email or newsletters. The “c” stands for “connectivist” and the course approach is typically that learners pursue their own learning outcomes with a focus on community and connections. xMOOCs, on the other hand, resemble traditional courses and more traditional higher education teaching methods are used. Pre-recorded video lectures and scalable forms of assessment are provided to learners who can interact in pre-set forums in a single platform rather than creating and/or sharing distributed content on the Web outside the platform.

Source: EU Commission (OpenCred study) 2016, Europe