Underway, or under way, is a nautical term describing the state of a vessel. "Way" arises when there is sufficient water flow past the rudder of a vessel that it can be steered. A vessel is said to be underway if it meets the following criteria:
If a vessel is adrift and not being propelled by any instrument or device, it is said to be underway, not making way. The concept of whether a vessel is, or is not, underway has important legal ramifications. For example, in many jurisdictions a child must be wearing a personal flotation device at the time the vessel is underway.
"Under weigh" is a variation, coming from folk etymology, first used in 1749. "Under way" is likely from the Dutch onderweg or Middle Dutch onderwegen (lit. "under" or "among the ways").