An Open to Buy (OTB) is the difference between planned purchases and merchandise that has been ordered from suppliers but not yet received. It functions like a merchant's checkbook, with planned purchases as the starting balance and purchase orders deducted as they are placed. Maintaining an appropriate OTB is important for ensuring sufficient inventory is available to meet planned sales goals. The document then defines related terms like planned purchases, on order, net sales, markdowns, markups, and others.