The availability heuristic judges the probability of events by the ease with which examples come to mind. Sometimes this heuristic serves us well, but the map is not the territory; the frequency with which concepts occur in your thoughts need not reflect the frequency with which they occur in reality. Undue salience, selective reporting, even subtle features of how the human brain stores and recalls memories can distort our perceptions about the probability of events. Because it is easier to recall words by their first letter, people judge words that begin with the letter r to be more frequent than words with r as their third lettter, even though in fact, the latter is more frequent. A second example is that selective reporting by the media of dramatic tragedies makes them seem more frequent than more threatening, albeit mundane, risks.
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External links
Availability heuristic at Psychology Wiki