Frequency illusion, also known as the Baader–Meinhof phenomenon or frequency bias, is a cognitive bias in which, after noticing something for the first time, there is a tendency to notice it more often, leading someone to believe that it has a high frequency of occurrence. It occurs when increased awareness of something creates the illusion that it is appearing more often. Put plainly, the frequency illusion is when "a concept or thing you just found out about suddenly seems to crop up everywhere."
The name "Baader–Meinhof phenomenon" was derived from a particular instance of frequency illusion in which the Baader–Meinhof Group was mentioned. In this instance, it was noticed by a man named Terry Mullen, who in 1994 wrote a letter to a newspaper column in which he mentioned that he had first heard of the Baader–Meinhof Group, and shortly thereafter coincidentally came across the term from another source. After the story was published, various readers submitted letters detailing their own experiences of similar events, and the name "Baader–Meinhof phenomenon" was coined as a result.
The term "frequency illusion" was coined in 2005 by Arnold Zwicky, a professor of linguistics at Stanford University and The Ohio State University. Arnold Zwicky considered this illusion a process involving two cognitive biases: selective attention bias (noticing things that are salient to us and disregarding the rest) followed by confirmation bias (looking for things that support our hypotheses while disregarding potential counter-evidence). It is considered mostly harmless, but can cause worsening symptoms in patients with schizophrenia. The frequency illusion may also have legal implications, as eye witness accounts and memory can be influenced by this illusion.
As you might guess, the phenomenon is named after an incident in which I was talking to a friend about the Baader-Meinhof gang (and this was many years after they were in the news). The next day, my friend phoned me and referred me to an article in that day’s newspaper in which the Baader-Meinhof gang was mentioned.