The Baader-Meinhof effect, also known as the frequency illusion, is a cognitive bias where after noticing something for the first time, there is a tendency to notice it more frequently in the future. This phenomenon occurs because the brain gives more attention to the new information, making it seem like it appears more often than it actually does. It is a result of selective attention and confirmation bias, where the mind focuses on the new information and confirms its presence more frequently.