classis

Classis, classis, f., third declension i-stem: ablative classe, genitive plural classium. The root is the one behind calo, "to summon", so a classis is fundamentally a body of men called up.

Servius Tullius is said to have sorted the citizens into property classes for the levy, and that is the first sense - hence English class and, from classicus, "of the top class", both classic and classical. Because the crews of the war galleys were raised by levy, the word slid across to mean the fleet itself, and in the historians that is what you will meet nine times in ten. A third sense, an army on land, survives in poetry and can catch you out.