1. The end of the world; judgment day. Every year, another nut job comes on the air, talking about how we're coming close to the day of doom and that we must all repent our sins.

2. By extension, any moment characterized by catastrophe, disaster, or complete ruination. The day of doom in my life was the day my daughter was killed by a drunk driver. It was a day of doom on Wall Street, as the property bubble burst and the economy plummeted to historic lows.