That is a big word, but do we really understand what it means?
Debate is contention in argument, strife, dissension, quarrelling, [controversy][, especially a formal [discussion] of subjects before a public assembly or legislature, in [Parliament] or in any deliberative assembly.
Wiki also offer definitions and examples of various forms of debates, including online.
It clarifies that debate can be formal and informal.
Informal and forum debate is relatively common, shown by TV shows.
Dictionary.com also offers a definition for "Debate"
a discussion, as of a public question in an assembly, involving opposing viewpoints
Today, a form of intellectual self-masturbation in which one attempts to scale an ivory tower in the shortest time possible. Policy debate was founded in 1957 in an attempt to disguise the throes of the real world with dignified, "intellectual" switch-side arguments. The framers intended it to allow young people to become more active in the world around them and encourage them to do something to help the ailing world. Instead, debaters made the activity into a competition of who could spew the most statistics, impacts, horribly damnable effects/results, screaming evidence at cutthroat speed, and modern debate sprouted as a result. Debate resides close to the top of the proverbial "ivory tower", as the real world gets translated into evidence and impacts, statistics and "strats" to win tournaments. Those who participate for the fun of it are exempt from this definition; it is only applicable to debate when it is applied as a critical theory that can "benefit the world".
So bottom line - debating what it means to debate is really a waste of time. We can argue that any formal and informal discussion can be classified as a debate if we take a more casual definition.