Not showing or having a tendency to return to a fixed position; lacking stability or equilibrium.
From 'a-' (without) plus 'static.' Used in physics to describe systems that don't naturally return to rest, borrowed in medical terminology to describe neurological conditions.
In physics, an 'astatic needle' was a revolutionary compass design—instead of always pointing north like regular needles, it stayed put wherever you turned it, solving a major 18th-century navigation problem.
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