When using a compass, what does the term "variation" refer to?

The term "variation" specifically refers to the angle between true north and magnetic north. This angular difference is crucial for navigation as it indicates how much the reading on a magnetic compass deviates from true north, which is the direction along the Earth's surface towards the geographic North Pole. Understanding variation is essential for accurate navigation, especially when plotting courses on nautical charts that are based on true north, as navigators need to adjust their compass readings to account for this difference in order to reach their intended destinations.

While the other options pertain to aspects of navigation and compass use, they do not accurately define "variation." For instance, the difference in compass readings may refer to the discrepancies due to local magnetic anomalies, while changes in compass accuracy over time relate to the calibration and maintenance of the compass. Magnetic interference involves local magnetic fields affecting compass readings and is not a direct measure of the relationship between true and magnetic north. Understanding these distinctions is vital for effective maritime navigation.