A histogram is a graphical illustration of a frequency distribution of a data set that contains numerical data. Histograms have several defining characteristics.

• The data is grouped into specific ranges of values known as intervals.
• All intervals in a histogram must be the same size.
• Interval data is marked in groups along the horizontal axis.
• A histogram is a collection of rectangles drawn above intervals.
• The height of each rectangle is proportional to the frequency of the data in the corresponding interval.

Consider an example situation. A fruit store wants to examine the weights of the apples they sell. To see the distribution, it is not necessary to show each apple's weight individually. Instead, the apples can be grouped by their weights in intervals of $10:$ $70\text{–}79\text{g},$ $80\text{–}89\text{g},$ and so on. A histogram looks similar to a bar graph. The difference is that a histogram has numbers on the horizontal axis and the bars cannot have a space between each other because the data is continuous.