In Return Path Platform and Inbox Monitor, a campaign is any email deployment to the Return Path Seed List.  Each deployment to the seed list will be counted against your monthly campaign limit.

If you send an email to the seed list, then make changes and re-send it to the seed list, it will be counted as two campaigns. This would also be the case if you sent the same email to two different segments of your database and included the seed list in both lists.