For most journals, do peer reviewers expect common notions such as arithmetic mean to be formally defined?
You do not need to define common &quot;notions&quot; such as the mean.
You do need to define common symbols, such as &quot;μ&quot;, because they are common symbols for more than one thing.
These are just broad guidelines.  Each research community has its own practices.  If unsure, avoid ambiguity.