APA Dictionary of Psychology

a graphical representation of a frequency distribution that is shaped more or less like the letter U, with the maximum frequencies at both ends of the range of the variable. For example, the number of people infected by the flu each year may have a U-shaped distribution by age, with those who are very young or very old having the highest frequency of occurrence. In an inverted U-shaped distribution, the most frequent values are in the middle of the distribution and the least frequent values are at the extremes.