The Baader-Meinhof phenomenon, also known as the frequency illusion, is a psychological effect that causes newly-discovered concepts to seem to pop up in an observer's life with increased frequency. It is a combination of selective attention and cognitive bias that appears to increase the likelihood of an occurrence.
The phenomenon occurs when someone learns of a new concept, person, place or thing. Since this new information remains fresh in the short-term memory, any new occurrences of that information are highlighted by the brain and tend to stand out against the background noise. For example, someone might encounter music from a particular band many times without recognizing it, but if a friend recommends that band or something else happens to highlight them in some way, any subsequent appearances acquire greater importance in the mind. This occurrence is due to the brain's natural tendency to discover patterns, and even to create and suggest patterns when none actually exist.
The name of the phenomenon comes from the Baader-Meinhof terrorist organization that was active during the 1960s and 1970s. The coiner of the phrase "Baader-Meinhof phenomenon" named the effect after learning of the group's existence and then seemingly running into mentions of their activities frequently afterward.