A sudden death of a band member (usually the lead singer) that leads to the band's permanent break up. They will decide that the member that died was irreplaceable and thus cannot go on.

Please note that there are plenty of bands, like the Rolling Stones, that carried on after the loss of a bandmate. Sometimes the surviving members form a new band, like how the surviving members of Joy Division reformed as New Order when Ian Curtis killed himself.

Queen didn't disband when Freddie Mercury passed away and John Deacon left, they just changed as a band.

The Doors was kinda this because they released two shitty albums after Jim Morrison died.