Definition of Main event
If we're talking about the Main event, the Main event of the Main event is the annual tournament played in Las Vegas that is the main event of the World Series of Poker. It is the $10,000 no-limit hold'em tournament that crowns the world champion of poker. It is the most prestigious tournament there is. But the term Main Event only means "main event" and can be used in any tournament series. It is a term used only in poker tournaments. Some poker series (casino or online) have dozens of events and the series usually concludes with a Main Event, a high buy-in tournament that caps the event. This is the event with the most media coverage.
Particularity of a Main event
Main events are, most of the time, the most interesting tournaments to play. First of all, the cash guarantee for the payouts will always be the highest. Secondly, the structure of the main events is always more interesting than the structures of the regular series tournaments. You will often have a huge starting stack, the blinds will increase very slowly, allowing you to take your time and really play deepstack poker.