Basic Definition:
A histogram is the graphical representation of value ranges (typically on the x axis) versus the number of values in each range (typically on the y axis). The value for each bin represents the number of measurements falling into the specified value range for that bin.
In CT image data, a histogram is often used to show the distribution of intensity values along the grayscale spectrum. It can be used to examine image contrast, noise distributions, set segmentation thresholds, and so on.