An installment contract is a contract which provides for multiple deliveries or acceptances of goods. An example would be if a restaurant ordered 100 pounds of meat to be delivered three times a week for six months.
The Uniform Commercial Code in the United States treats an installment contract as a single indivisible contract, even if the parties agree that each delivery will constitute "a separate contract." This is mainly important in determining when a breach of contract occurs. In the above example, if the supplier only delivered 95 pounds of meat on one of the designated days, it would not constitute a material breach of the contract (and therefore the restaurant would not be entitled to terminate the contract) because the entire deal, to make 78 deliveries, is not substantially impaired.
Note that this rule does not apply to service agreements, which are generally governed by common law in the US and therefore follow different rules with respect to material breach.