From Longman Dictionary of Contemporary Englishrulerrul‧er /ˈruːlə $ -ər/ ●●● W3 noun [countable] 1 PGOsomeone such as a king or queen who has official power over a country or area► see thesaurus at leader2 
DHHa long flat straight piece of plastic, metal, or wood that you use for measuring things or drawing straight lines a 12-inch ruler
DHHa long flat straight piece of plastic, metal, or wood that you use for measuring things or drawing straight lines a 12-inch rulerExamples from the Corpus
ruler• Both rulers and ruled may believe that some issues are not a matter for public concern.• They had been imprisoned there by a long dead ruler, Llud.• When his worship spread to a town where there was already a divine ruler the two were slowly fused into one.• No earthly ruler dared interfere with the civil life of his church and kingdom.• Despite his arrogance and cruelty towards opponents, he had been a remarkably enlightened and often liberal and generous ruler.• Some regimes are governed by hereditary rulers.• Ramses II, ruler of Egypt in 13,000 BC• King Priam was a firm, but just ruler.• Mr Obasanjo was running the country on that occasion too, but as a military ruler.• Several countries have condemned Nigeria's military rulers for human rights abuses.• All the accounts I have examined agree, more or less, that Zeus is the ruler of the cosmos.• The rulers promptly monopolized it for their own regalia and as a medium for bestowing honour and obligations on their retainers. 