Technically the flight is overbooked, more tickets sold than seats. A flight is considered "oversold" when more passengers show up then there are available seats. Overbooked flights frequently do not go out full due to a variety of reasons

A. Missed connection due to weather and/or delays
B. Not showing up for a flight
C. Last minute cancellations

That is why most airlines overbook. Depending on the route, time of week, time of day, and travel season the amount overbooked varies.