GW150914 was the first gravitational-wave event detected from merging black holes.
The Laser Interferometer Gravitational-Wave Observatory, or LIGO, detected the signal on 14 September 2015. The waves came from the final inspiral and merger of two black holes, each roughly a few dozen times the Sun’s mass. The merged object was also a black hole, and some of the system’s mass was converted into gravitational-wave energy.
The event was announced publicly in February 2016. Its waveform closely matched numerical-relativity predictions, providing the first direct detection of gravitational waves and the first observation of a binary black-hole merger.
The name records the date in year-month-day form: 150914 means 2015 September 14. GW170817, detected later, came from merging neutron stars rather than two black holes, so it is a different kind of event.