This is a definition of computer-mediated communication that I've been using for several years; it is not meant to be the definition; I've used this as a working statement to explore the dimensions of what computer-mediated communication is and how to approach its study. Since I do research on Internet-based CMC, this definition is oriented to that context; but I don't mean to imply here that all CMC is Internet-based.
I believe that process and context are key themes in the study of computer-mediated communication.
Computer-Mediated Communication (CMC) is the process by which people create, exchange, and perceive information using networked telecommunications systems (or non-networked computers) that facilitate encoding, transmitting, and decoding messages. Studies of CMC can view this process from a variety of interdisciplinary theoretical perspectives by focusing on some combination of people, technology, processes, or effects. Some of these perspectives include the social, cognitive/psychological, linguistic, cultural, technical, or political aspects; and/or draw on fields such as human communication, rhetoric and composition, media studies, human-computer interaction, journalism, telecommunications, computer science, technical communication, or information studies.