Histogram is a graphical display of the data distribution between non-overlapping intervals within the range of the data consisting of the count (or number) of values that fall into each interval.
These counts can be divided by the total number to give a relative frequency of occurrence. Bars are plotted with height proportional to the relative frequencies.
von Storch, H., and Zwiers, F.W., 1999. Statistical Analysis in Climate Research. Cambridge University Press. Pages 80-81.