Principal clock

Principal clock: Of a set of redundant clocks, the clock that is selected for normal use.

Note 1: The principal clock may be selected because of a property, e.g., superior accuracy, that makes it a unique member of the set.

Note 2: The term "principal clock" should not be confused with, or used as a synonym for, the term "primary frequency standard."

Source: from Federal Standard 1037C and from MIL-STD-188