The act of deciding which channel/touchpoint deserves credit for a conversion.
A customer sees your Facebook ad, searches Google, clicks an organic result, reads a blog, gets a retargeting ad, and finally buys. Which channel deserves credit? That's the attribution problem. Common models: first-touch (FB ad gets credit), last-touch (retargeting gets credit), linear (all equal), time-decay, position-based, and data-driven. Every model is wrong in some way.
Attribution tells you where to invest. Under-investing in early-funnel channels (because last-touch undercounts them) is the most common attribution mistake. Cross-check attribution models; use MER as the reality check; don't bet the business on one dashboard's view.