A (usually rigid), flat, rectangular measuring or drawing device with graduations in units of measurement; a straightedge with markings.
"She used her metal ruler to draw perfectly straight lines on the blueprint."
In plain English: A ruler is an official person who has power over a country, group of people, or organization.
"The student used his ruler to draw straight lines on his paper."
Usage: The noun form is often confused with "rule," but they are distinct words with different meanings and pronunciations. When used as a verb, the term specifically refers to corporal punishment rather than measuring or drawing.
To beat with a ruler (as a traditional school punishment).
"The strict teacher made the student kneel and beat his hands with a wooden ruler as punishment for talking in class."
In plain English: To rule means to be in charge of and make all the decisions for a group of people or an area.
"The government ruler with strict laws for years."
The word ruler comes from Middle English reuler, which was formed by adding the suffix -er to the verb rule. This construction created a noun meaning one who rules or governs, reflecting its current definition without any shift in core sense over time.