OMS (Order Management System)
Definition
Software that collects orders from all your sales channels, routes them to the right warehouse and keeps inventory in sync across channels.
Want a deeper explanation?
An OMS becomes important once you sell in several places from shared stock. It prevents overselling and gives one view of orders.
Its inventory data is only useful to accounting if it ties to your books. Watch for: OMS, 3PL and accounting showing three different inventory numbers with nobody reconciling them.
Example
Orders from Shopify, Amazon FBM, Walmart and wholesale flow into one OMS, which sends each to the right 3PL and updates stock everywhere.