Cross section data is drawn at a single point in time as a sample of many different units from a certain population.
Cross section data is drawn at a single point in time as a sample of many different units (e.g. individuals, households, firms) from a certain population. The name for this type of data was borrowed from geometry where a cross section is the shape which is obtained by cutting straight across an object. Since there is no time dimension, cross sections are static in nature and are therefore sometimes referred to as "snapshot of the population".
When a cross section is repeated over time, but not with the same units, it is called a pooled cross section.
Examples of cross sections and pooled cross sections are the Canadian Integrated Criminal Court Survey and the U.S. Current Population Survey, respectively.