“Bottleneck” and “constraint” are terms used to describe limitations or restrictions in processes, and they are often used interchangeably. A bottleneck is always the slowest activity in a sub-process that, at a given time, cannot deliver what the customer demands. The bottleneck can shift depending on the time period or the product/product group/mix, and can vary over time. Machines, employees, and IT systems can also be bottlenecks.

A constraint is a general limitation that affects the performance of the system, such as the availability of employees or materials, which may be restricted by company policies or contracts. The Theory of Constraints (TOC), developed by Eliyahu M. Goldratt, is a management philosophy that identifies constraints as the key factors for improving overall system performance. When one constraint is resolved, a new constraint emerges.