Short definition
Attribution is the method of assigning credit to the marketing channels and touchpoints that produced a lead or sale.
In depth
Attribution answers the most expensive question in marketing: which dollars actually produced revenue. Common models include first-touch (credit the channel that introduced the customer), last-touch (credit the channel right before conversion), linear (split credit evenly), and data-driven (algorithmically assigned). Service businesses without attribution often double down on the wrong channel. Solid attribution requires UTM tagging, call tracking, hidden form fields capturing source, and a CRM that stores the touch history per contact.
Example
A med spa believed Instagram was its top lead source. After adding call tracking and UTM-tagged links, attribution showed Google Maps was producing 3× the bookings of Instagram.