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If you Google the phrases "magnetic declination" and "magnetic variation", you'll find that have similar numbers of hits. Articles focused on navigation prefer "variation". Articles in geophysics and geology prefer "declination". So we're stuck with two fairly useless names for the same thing. And it's been this way for over a century...

There's plenty of linguistic fun in navigation since it has ancient roots and modern developments. You can see deep into the history of navigation by comparing two terms:
  Declination, which is the latitutude of the sub-star point for any celestial body, and,
  GHA, which is the longitude of the sub-star point.

One is an ancient word with deep linguistic roots, and yet it really doesn't explain anything to a modern student: what exactly is "declining"? Well, nothing. It's just an odd, old word. Meanwhile, GHA is so modern that it's an acronym, which basically guarantees it originated in the 20th century. It's sufficient and accurate to say "it's just a name for longitude" (and that's how I usually teach it, at least at the start of a lesson). But if you break it down and explain in detail what "Greenwich Hour Angle" is, at least it's a moderately explanatory expression. Why isn't there a word as old as "Declination" to indicate the longitude of the sub-star point? Aren't these parallel concepts? Things that should have originated in science at the same time?? Well, of course, it's because, until about 250 years ago, latitude was easy and longitude was hard and seen as something really quite different from latitude. And all that history is implied right there in those two navigational terms...

Frank Reed