Connecting more users to a system than can be fully supported if all of them were using it at the same time. Networks and servers are almost always designed with some amount of oversubscription, counting on the fact that not everybody needs the service at maximum speed simultaneously. If they do, delays are certainly the result, and outages may occur.
Also called "overbooking," oversubscription is common in the airline industry where more seats are sold than are available. The airline is expecting that some passengers will miss their flight or cancel at the last minute.