nox

nox, noctis is a third declension feminine noun: ablative nocte, plural noctes, noctium, noctibus. The x of the nominative is c plus s, which is why the stem underneath is noct- and every other form shows it.

The ablative nocte on its own means "at night" or "by night", with no preposition needed, exactly as die means "by day". "The whole night through" uses the accusative of duration, totam noctem, while tota nocte sets it as the period within which. It is one of the oldest words in the family and looks it: Greek nyx, German Nacht, English "night" all sit alongside. It feeds nocturnus, aequinoctium and the Romance notte, noche, nuit.