In the new draft OpenStreetMap Contributor Terms, an 'active contributor' is defined as:
a contributor (whether using a single or multiple accounts) who has edited the Project in any 3 calendar months from the last 6 months (i.e. there is a demonstrated interest over time) ...
This is a visualisation of the edits of the 8173 'active contributors' (as of 1/2/2010) across the last three years. For each contributor a mark is placed along a horizontal line under each day that they contributed an edit. At the top are contributors with almost daily edits (within the last six months) and, at the bottom, those that only just meet the criteria. The vertical axis has been scaled from 8173 to 1600 pixels. There is a fairly strong correlation between recent and historical edit intensities.
This was calculated from the planet changeset dump, and assumes that a changeset was edited on every day if it was open for more than one day (rare). It regards each user id as an individual, so does not take account of a contributor using multple user accounts.