Exchange money or some other type of in-game currency for an item or service (enchant, recipe, training, etc.). This includes a winning bid or buyout at an Auction House or obtaining something from a vendor or another player character (trading for money or by mail via COD). Also sometimes referred to as a "purchase".
Items you can buy is almost always BoE (or just not soulbound) and are generally easier to obtain than loot or quest rewards. A recipe for a profession, for example, is generally easier to find if it can be bought. The downside is that purchaseable items are generally less powerful (an extreme case being end-game armor/weapons) than BoP items due to their wide availability.