Definition of downswing
A downswing is when you accumulate losing sessions over a short, medium or long period of time. There is no rigid definition of a downswing, but if you are a winning player who is used to having winning sessions the majority of the time and you have 5-6 losing sessions due to terrible beatbeats, you can say that you are downswinging. Downswings happen for tournament players, sit'n'gos and cash games alike. And you will experience more downswings in some variants than others. For example, you will have more downswings in tournaments than in cash games and you will have more downswings in Omaha than in Hold'em.
Winning players and downswing
A bad player who has 20 bad sessions would not necessarily be downswinging. If you are a long-term losing player and you are not playing optimally, losing over a long period of time will not be downswing, but the consequence of your bad decisions. To call it downswing, you must have evidence of being a winning player. Downswing can be short (less than 10 losing sessions), medium long (losing 40 of your last 50 sessions) and long (downswing is a year or two). Downswings are inevitable. Even the best players in the world experience downswings.