The ruler of a country is the person who rules the country.
双语例句
例：
The former military ruler of Lesotho has been placed under house arrest.
莱索托前军事统治者已被软禁了。
2.?[C 可数名词] 尺
A ruler is a long, flat piece of wood, metal, or plastic with straight edges marked in inches or centimetres. Rulers are used to measure things and to draw straight lines.