Whenever I hear that in strategy meetings, or read it in books and on websites, I feel a shudder go down my spine. Breakage is something manufacturers refer to when running a rebate program, or selling gift cards, and it refers to the number of people who will redeem them. That's the breakage.
In short, it's "how many people will actually take us up on this offer and cost us money?" It's awful. Breakage is used in promotions to lure people in, in the hopes that they will pay full price for a product they had no intention of buying, but are because of the huge rebate, then forget to send it in.
Oh, and if they do find the time, it involves jumping through more hoops than the dogs at Crufts. If you use breakage in your campaigns, shame on you.