Prox is short for the Latin term "proximo mense" and means in the next month. Prox terms is an invoicing agreement that requires the receiver of a good or service to reimburse a business within a month.
Oftentimes a business will provide discounts if payment is received prior to a month from the invoice due date. Businesses provide discounts to improve their cash flow and their ability to repay debts or to fund payroll. The discount terms and payment due date are stipulated on the invoice.
Seasonal businesses often have extended invoicing terms with their customers. This encourages customers to take shipment of products prior to when they will be able to resell them.