The shut down of one or more servers connected to many other servers as a network.
Net Splits may be likened to elementary school cold outbreaks: suddenly, many of your classmates are not at school but will eventually return.
On IRC, the dissapearance of a large number of users due to a server or connection failure. Due to the connected nature major IRC networks, when one server of the network fails or disconnects from the others, all users on that server dissapear with it (from the point of view of the rest of the IRC network). This gives the appearance of a sudden and mass exodous in popular channels.
Crackers once used one of many types of attacks to shut down a server and then, upon restoration, they would gain “op” status in the channel of their choosing. This can inconvinience hundreds or even thousands but IRC networks are now less vulnerable to such attacks.
Related
TakeDown.NET -> “Net-Split”