A histogram is a graphical representation of the distribution of numerical data. It is a type of bar chart that shows the frequency or number of observations within different numerical ranges, called bins. The bins are usually specified as consecutive, non-overlapping intervals of a variable. To construct a histogram, the first step is to "bin" (or "bucket") the range of values, divide the entire range of values into a series of intervals, and then count how many values fall into each interval. The height of each bar represents the frequency or number of observations in each bin.

Histograms are useful for detecting outliers and/or gaps in the data set. They are often used in statistics to visualize the shape of data distribution across a range of values. A histogram can be used to communicate the distribution of data quickly and easily to others. It is considered one of the seven basic quality tools.

A histogram can be thought of as a simplistic kernel density estimation, which uses a kernel to smooth frequencies over the bins. This yields a smoother probability density function, which will in general more accurately reflect the distribution of the underlying variable. The density estimate could be plotted as an alternative to the histogram, and is usually drawn as a curve rather than a set of boxes.

In summary, a histogram is a graphical representation of the distribution of numerical data, where the height of each bar represents the frequency or number of observations in each bin. It is useful for detecting outliers and/or gaps in the data set and is often used in statistics to visualize the shape of data distribution across a range of values.