Quartile is a statistical measure that divides a dataset into four parts, each containing an equal number of observations. In Excel, you can use the QUARTILE function to calculate the quartile of a dataset. The syntax for the function is '=QUARTILE(array,k)', where 'array' is the range of data you want to analyze and 'k' is the quartile you want to calculate (1 for the first quartile, 2 for the second quartile, etc.).