In computer networking, mainly Internet Relay Chat (IRC), netsplit is the event when two servers disconnect. Because how IRC networks are built, a disconnection between two servers splits the entire network into two parts. This makes it look like people on the other half of the network has quit from IRC.
A normally functioning network: Bob, Joe and Sara can all see each other.
A network on which a netsplit has occurred: Bob sees Joe quitting, Joe sees Sara and Bob quitting.