APA Dictionary of Psychology

in studies of visual search, the situation in which a search for the presence of a feature produces one pattern of results but a search for the absence of that feature produces another. For example, searching for a Q in a field of Os (i.e., searching for the “tail” segment in the Q) is relatively easy, but searching for an O in a field of Qs (i.e., searching for the absence of this segment) is difficult.