An Etymological Dictionary of Astronomy and Astrophysics
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Relating to a constraint or system that contains time explicitly. For example, a pendulum with an extensible string of length l rheonomous, the condition of constraint is: x² + y² = l²(t), where l(t) is the length of the string at time t.

From Gk. rheo-, from rheos "a flowing, stream, current," → rheology, + -nomous, → -nomy.