Flow rate / throughput: The number of flow units (e.g. customers, money, produced goods/services) going through the business process per unit time, e.g served customers per hour or produced parts per minute. The flow rate usually is an average rate.
Flow time: The amount of time a flow unit spends in a business process from beginning to end, also known as the total processing time. If there is more than one path through the process, the flow time is equivalent to the length of the longest path.
Inventory: The number of flow units that are currently handled by a business process, e.g. the number of customers in a store, the number of enrolled students in an university etc. pp.