Variation is the enemy of processes. Variation in the inputs or in the process itself causes variation in the output of the process. And that variation is perceived as poor quality.
The layman’s definition of variation is simply fluctuation.
More advanced definitions of variation categorize it in two ways. Common cause variation (also called noise) is the variation inherent to the system. Special cause variations are the external, unexpected factors that pop up from time to time and affect the output.
Mathematically, you will see common cause variation clustered in some sort of distribution around the mean. Special cause variation can be all over the map.
Both types of variation can, and should, be addressed through standardization (i.e. Standard Work).