PLAIN-LANGUAGE DEFINITION
Frequency
Average frequency is impressions divided by reach for the same scope.
IN PRACTICE
An example
1000 impressions across 500 reached accounts gives average frequency 2.
What this does not tell you
An average does not mean every person saw the creative exactly twice.
Use the term precisely
When you use this term in a brief or report, include the relevant source, scope and assumptions. An example explains the concept; it is not a benchmark or a promise of results.