Bolt
Bolt turns up in 2 fields: fasteners, maritime. Here is what it means to the people in each one.
Bolt in fasteners
A headed, externally threaded fastener meant to be tightened by turning a mating nut rather than its own head.
An externally threaded fastener with a head, designed to pass through clearance holes in the parts and be tightened by turning a nut. The difference from a screw is about intended use, not looks: the same hex part becomes a cap screw when it is driven into a tapped hole by its head. A bolt works as a stretched spring, so its job is to clamp the joint through preload rather than to act as a pin in shear unless the design says so.
"Those are bolts, not cap screws, so put a nut and a hardened washer on every one and turn the nut, not the head."
Bolt in maritime
A traditional standard unit of measurement for a roll of sail canvas.
A standardized roll of canvas used by sailmakers, traditionally measuring 39 yards in length. The width of the bolt varied depending on the weight and purpose of the canvas, but the length provided a consistent unit for ordering supplies and calculating the material needed for a ship's suit of sails.
"The sailmaker ordered ten bolts of heavy canvas to replace the main course after the storm."