In a now-classic study by Simons and Chabris (1999), study participants watched two teams pass a basketball among teammates. One team was wearing white shirts and the other was wearing black shirts. Participants were told to count how many times the team wearing the white shirts passed the basketball, More than half of the study participants failed to notice when a man wearing a black gorilla suit walked right past the two teams. This phenomenon is known as .

Inattentional blindness.

Explanation:

Inattentional blindness occurs when we fail to notice something that is in our visual field because we are focussing our attention on something else. This is useful when we want to concentrate on a task, and to carry it out, we have to "ignore"  all the different stimuli that are around us and that do not help with the task.

The gorilla in the study was a stimulus that wasn't important for the task, so the participants didn't pay attention to it.