Engineering tolerance is the permissible limit or limits of variation in:

1. a physical dimension,
2. a measured value or physical property of a material manufactured object, system, or service,
3. other measured values (such as temperature, humidity, etc.).
4. in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel (see structure gauge and loading gauge).
5. in mechanical engineering the space between a bolt and a nut or a hole, etc..

Dimensions, properties, or conditions may have some variation without significantly affecting functioning of systems, machines, structures, etc. A variation beyond the tolerance (for example, a temperature that's too hot or too cold) is said to be non-compliant, rejected, or exceeding the tolerance. If the tolerance is too restrictive, the machine being incapable of functioning in most environments, it is said to be intolerant.