At times I’ve used the words Preference and Priority interchangeably to describe the numerical value in MX records that determines which record gets used first. Many folks do it, and no, it’s not a crime to use either as long as the message gets across.
However, when actually committing words to paper – when writing technical articles, white papers, books, etc., use of the correct technical term is a requirement that cannot be ignored.
RFC 1035 Domain Names – Implementation and Specification says it’s PREFERENCE – “A 16 bit integer which specifies the preference given to this RR among others at the same owner. Lower values are preferred”.
I will try to stick to Preference when talking about MX records.