A category killer is a retailer, often a big-box store, that specializes in and carries a large product assortment of a given category.^{[1]: 109–111} Through their wide merchandise selections, low pricing, deep supply, large buying power, and market penetration, they have a comparative advantage over other, smaller retailers, and can greatly reduce the sales of rival retailers within that category, in the area^[2] and beyond it.^[1]

In essence, they are a price- or discount-based specialist mass-retailer.^{[1]: 13 [2]} Chains such as OfficeMax, Best Buy, Barnes & Noble, and Hobby Lobby have been considered category killers.^[3]

Once typically found in power centers, increasingly they are found in or adjacent to (as an outbuilding of) repurposed traditional malls.

Large category killer stores are mostly in mid- and large-sized cities, because a large population is required to be feasible.^[1]: 33

Local merchants in cities with category killers "may suffer a substantial reduction in sales," and stores in a wider radius can be affected by the draw.^{[1]: 109–111} Between 1983 and 1993, Iowans spent 31% less in hardware stores, translating to a loss of 37% in the same time to those stores as a result of category killer stores.^{[1]: 67–68, 89}

Sporting goods stores that are category killers range in footprint from 10,000 to 40,000 square feet (930 to 3,720 m²).^[1]: 37Home Depot carries 30,000 items in 100,000-square-foot (9,300 m²) stores.^[4]: 148

• Big-box store