Admiral is the highest rank in a navy. The term is used internationally by many countries. It derives originally from the Arabic word emīr (= commander), and came to English by way of Old French and Latin.
Variations on the term are:
Admiral of the Fleet: originally, the Admiral in charge of an operational fleet, later used to designate senior status.
Vice-Admiral: step lower in rank than Admiral.
Rear-Admiral: two steps lower in rank than Admiral.