For most journals, do peer reviewers expect common notions such as arithmetic mean to be formally defined? If making use of standard statistical notions such as arithmetic mean, do peer reviewers still expect to see a formal definition such as μ = ( Σ Xi ) / N, or is the notion so common that it'd be redundant to define it this way?
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.