Definition
A category of navigation and procedure-design problems that arise because Earth's magnetic field is not aligned with true north and shifts over time, causing the difference between true north and magnetic north (magnetic variation) to vary by location and to change gradually year by year. These issues affect the accuracy of magnetic compass readings, the published magnetic courses on charts and instrument procedures, runway magnetic headings and numbering, and the alignment of navigation aids such as VORs, all of which must be periodically updated to reflect current variation values.
Plain English
Because magnetic north is not the same as true north, and because it slowly drifts over time, the magnetic headings shown on charts, runways, and instrument procedures can become inaccurate. These problems are the practical consequences pilots and procedure designers have to deal with as a result.
Context Anchor
Seen in instrument procedure design, chart updates, runway numbering, and discussions of why a published course or heading may change over time.
Derivation
Magnetic comes from the Latin magneticus, relating to the lodestone (a naturally magnetic mineral). Variation comes from the Latin variare, to change. Together the phrase points to the changing difference between magnetic north and true north -- a difference that varies both by location and over time.
Why Pilots Care
Failing to apply variation correctly can produce heading and course errors that lead to navigation deviations or missed approaches.
Analogy
It is like using two slightly different north arrows on two maps. If you do not know which one your directions are based on, following the same number can point you in a slightly different direction.
Grounding Statement
A compass points toward magnetic north, but charts and procedures must account for how far that direction is from true north at that place and time.
Intuition Check
Do not read variation here as a vague change or inconsistency. In this context, magnetic variation is a specific angle: the difference between true north and the north direction shown by a magnetic compass.
Example Sentence 1
The airport renumbered its runway from 16/34 to 17/35 because of issues related to magnetic variation, as magnetic north had drifted enough to change the runway's magnetic heading.
Example Sentence 2
The briefing highlighted issues related to magnetic variation that change over time and must be updated on approach plates.