p(doom) is the probability of an event called “doom” occurring. In statistics, the probability represents the likelihood of an event happening, and it usually ranges between 0 and 1. A value of 0 indicates that the event is impossible, while a value of 1 means that the event is certain to occur. In current AI parlance it’s a measure of people’s instinctive fear of humans being existentially threatened by the existence of AI.