Deviation is a gap between a desired state and actual, which is considered to be practically, commercially or for any other valid reason impossible to eliminate at given circumstances.
Synonym for the term “deviation” is exception
Example: A standard requires that you shall use an XYZ type paint for any paintwork in your house. However it is known that the standard is out of date and there is no such type of paint available in the current market. In this case you can assess the risks of using a different type paint and, if these are minimal, then you can live with the Deviation, otherwise you should think of any risk mitigation measures.