A bypass is a highway that avoids (passes by) a built-up area, town, or village, to let through traffic flow without interference from local traffic, to reduce congestion in the built-up area, and to improve road safety.

In the Interstate highway system in the USA, bypasses and loops are designated with a three digit number beginning with an even digit.

If there are no strong land use controls, buildings are built along a bypass, converting it into an ordinary town road, and the bypass may eventually become as congested as the local streets it was intended to avoid.

In medicine, bypass generally refers to bypass surgery, e.g. coronary artery bypass surgery, in which blood vessels, or less frequently synthetic tubing, is surgically moved/implanted in order to create an alternate additional route for blood flow.

In telecommunication, the term bypass has these meanings:
1. The use of any telecommunications facilities or services that circumvents those of the local exchange common carrier.
Note: Bypass facilities or services may be either customer-provided or vendor-supplied.
2. An alternate circuit that is routed around equipment or system component.
Note: Bypasses are often used to let system operation continue when the bypassed equipment or a system component is inoperable or unavailable.

In rebreather breathing sets, a bypass is a hand-operated valve that the user can use to let more oxygen (or other breathing gas) into the breathing system, by-passing the cylinder's flow rate control valve.

Source: from Federal Standard 1037C and from MIL-STD-188