An Etymological Dictionary of Astronomy and Astrophysics

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.

See also: From Gk. rheo-, from rheos “a flowing, stream, current,” → rheology, + -nomous, → -nomy.