Histogram
From The Glossary of Digital Photography
A graph displaying brightness along the bottom (x-axis) and the number of pixels falling within that brightness value along the vertical (y-axis). Black is displayed on the left; white is displayed on the right. Depending on the type of image, the best overall exposure will feature a bell-shaped curve in the center, tapering off in both directions to the edges of both sides. Images in which the curve runs either to the right or left and then drops suddenly have either the shadows or highlights clipped. Clipped means that the image exposure exceeded the range of the image sensor and the curve suddenly ends or is clipped off. If the curve is mostly on the left, the image is dark in tone; if the curve is mostly on the right, the image is light in
tone.
