A boiler is a large metal tank that heats water to create steam or hot water, usually by burning fuel like coal, oil, or natural gas. In older apartment buildings and houses, the boiler sits in the basement, heating water that flows through pipes to warm every room. Ocean liners like the Titanic had massive boilers that created the steam to turn their engines.

Before electricity became common, boilers powered factories, trains, and ships. Workers called stokers would shovel coal into the boiler's furnace all day to keep the fire hot enough. The steam built up tremendous pressure inside, which could turn wheels, run machinery, or push a locomotive down the tracks.

The expression boiler room sometimes refers to a place where hard, hot, unglamorous work happens. In modern buildings, boiler rooms are often cramped spaces in the basement where maintenance workers check gauges and keep systems running smoothly.

A related phrase is boilerplate, which originally meant the thick steel plates used to make boilers, but now refers to standard, repetitive text used over and over, like the same rules printed at the bottom of every permission slip.