interregnum comes from Latin interregnum ('an interval between reigns'), from inter- ('between') + regnum ('rule, kingdom'), from regere ('to rule'). Morphologically, the prefixed shape matters: the initial element belongs to the recorded formation and helps explain the modern form's surface analysis. In present English, the noun sense is "an interval between two reigns or governments; a pause in continuity", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as reign, regnant, regal, and interregnal. Its semantic neighborhood includes interim, interlude, hiatus, and interval, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin interregnum and Latin regere.