A cusec was a unit to measure distance used by Theela's species who inhabited the second planet of the Taurean system.

When it was discovered that the landing party from the USS Enterprise was hidden in a giant urn that was filling with rain water, Theela noted that the nearest water accumulation was many cusecs from the main compound. (TAS: "The Lorelei Signal")

No indication is given on how far is a cusec, but one must assume as being a considerable distance like a kilometer or a mile.
Though it is spelled "cusec," Theela pronounces it as "coh-seck."