What does deviation refer to in navigation?

Deviation in navigation specifically refers to the error introduced in a magnetic compass reading due to local magnetic influences on the vessel. This can occur from the ship's own magnetic field or magnetic objects onboard, which distort the compass reading. Therefore, deviation is essentially the difference between the compass heading and the true magnetic heading.

While compass headings provide a way of navigating, they must be adjusted for deviation to ensure accurate navigation. Understanding this concept is crucial for mariners since navigating solely by a compass without accounting for deviation can lead to significant errors in course plotting.

The other options, while relevant to navigation, do not specifically define deviation. The effect of currents relates to the vessel's movement through water, the difference from magnetic north addresses magnetic variation, and the effect of tide concerns changes in sea level but does not pertain to the compass directly.