On Wikimedia's official stats, active contributors are defined as registered users who have edited five or more times in a month (while logged-in). There are currently just under 100,000 active contributors (*) across all Wikimedia projects. People with 100 or more edits in a month are referred to as very active contributors.
Both thresholds were chosen arbitrarily, but have gained importance over the years, and are now generally seen as key metrics. Five edits a month was chosen as a lower limit to discern intentional editors from "passers-by," people who may only have edited to see how it works. In some media coverage, larger, all-inclusive counts are presented where any person with just one edit is included. This tends to distort public perception as often the raw number is quoted without defining the number used.
Nowadays wikistats publishes activity counts per wikipedia on many levels, for logged-in, registered users and bots alike. For example here are activity statistics for the English Wikipedia: 
(*) Users who edit on several wikipedias are at the moment still counted twice (since introduction of Single User Login this can theoretically been fixed). Also a non-negligible number of users have several accounts (so called sock puppets).