sealed vessel where water is converted to steam
"The old boiler in the basement began hissing as it heated the water into steam for the radiators."
A person who boils something.
"The lawyer used boilerplate language in the contract that applied to every client without any changes."
Boilerplate.
In plain English: A boiler is a machine that heats water to make steam for power plants or hot water for homes and businesses.
"The boiler broke down during winter, leaving us without hot water for several days."
The word boiler comes from combining the verb boil with the suffix -er. It originally referred to a person or thing that boils and now describes an apparatus used for heating water by boiling it.