The angle between true north and magnetic north at a given place, the difference between a magnetic and a true course.
Magnetic variation (also declination) is the angle between true (geographic) north and magnetic north at a particular place. A compass points to magnetic north, charts are drawn to true north, and variation is the correction between the two. It's called east when magnetic north lies east of true north, west when it lies west.
Where to find it
Variation depends on where you are and slowly changes over the years as the Earth's magnetic field drifts. The compass rose on a chart gives the value for a reference year and the annual change, so you can bring it up to date. GPS devices and plotters calculate it from a built-in model.
Applying it
True course = magnetic course + variation (east positive, west negative)
With 4° east variation, a magnetic course of 086° is a true course of 090°. Going the other way, from chart to compass, you subtract east and add west. The old memory aid: variation east, magnetic least; variation west, magnetic best. With east variation, the magnetic figure is the smaller one.
Variation is the same for every boat in the same place and on every heading. The error caused by the boat itself is deviation. Both together lead from the compass reading to the true heading.