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Is there a semantic difference between engine and motor? In some cases, would the use of one or the other word be technically incorrect?

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I like the definitions provided by WordNet and their definition is the way I've understood the difference:

  • A motor is a machine that converts other forms of energy into mechanical energy and so imparts motion.

  • An engine is a motor that converts thermal energy to mechanical work.

wordnetweb.princeton.edu

So an engine is a specific type of motor. That's why it's not incorrect to speak of a motorboat, or a motorcar, or a motor speedway, even if the boat or car is clearly powered by combustion.

Note that if there's no combustion, there's no engine. Purely electric cars don't have engines.

As nouns, motor can also refer to a nonspecific agent that causes motion: "happiness is the aim of all men and the motor of all action", and engine can refer to something used to achieve a purpose: "an engine of change", a railway locomotive, or a machine used in warfare: "medieval engines of war".

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  • I like this one the best so far, but solid rocket motor confuses this answer, is this the exception to this rule?
    – user10126
    Sep 15, 2011 at 21:58
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    @Jarrod I'm confused, exception in what way? All engines are motors…
    – ghoppe
    Sep 15, 2011 at 22:02
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I picked the relevant parts from my dictionary and would say that these terms have been (over)used figurative.

Googles services are called a search-engine but never would be called a search-motor. In the field of software development we talk about graphic-engines, physical-engines etc. but never about motors.

motor |ˈmōtər| noun a machine, esp. one powered by electricity or internal combustion, that supplies motive power for a vehicle or for some other device with moving parts. • a source of power, energy, or motive force : hormones are the motor of the sexual functions.

engine |ˈenjən| noun a machine with moving parts that converts power into motion. • a thing that is the agent or instrument of a particular process : exports used to be the engine of growth.

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In a strictly physics sense an engine converts energy into mechanical work.
A motor is a subset of engines that produce motion as the mechanical work.
So all motors are engines but not all engines are motors.

Sometimes they are used interchangeable eg. rocket engine and rocket motor. Motor is generally used for electrical devices but it's also used as an abbreviation for motorized eg. motorboat or motorcycle.

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    Interesting that WordNet reverses the set inclusion: an engine is a type of motor. I suppose it depends on your point of view: all mechanical work is motion, strictly speaking.
    – ghoppe
    Sep 15, 2011 at 21:46
  • @ghoppe - motive is probably a better word.
    – mgb
    Sep 15, 2011 at 21:46
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    There's a motor in my electric fan, but I wouldn't say it has an engine.
    – Peter Shor
    Sep 16, 2011 at 2:17
  • @ghoppe you may need mechanical work just to remain in place. Think of an airplane flying against the wind
    – Dr. belisarius
    Sep 16, 2011 at 4:00
  • Its actually the opposite. The engine is a subset of motor. Not the other way around.
    – user151513
    Mar 30, 2022 at 2:35
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The real difference, is the fact that "motors" run on electricity, while "engines" run on combustion. However, many people interchange the usage of these words:

On the rare occasions we encounter one, we refer to a steam locomotive as an engine, the same word that we give to the motive power of an aircraft. But all electrical devices are driven by motors. In Britain at least, one’s personal transport is a motor car (with compounds such as motor trade, motor vehicle and motor sport), even though it’s always powered by an engine. Small boats may have outboard motors and then are often called motor boats.

From a mechanic:

So, what is the difference between an engine and a motor?
Motors are electric. Electricity powers motors.
Engines are powered by come sort of pressure or combustion, ie: steam engines, automobile engines, diesel engines.
Pretty simple, really.

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    Then why are race tracks called Motor Speedways instead of Engine Speedways? This goes back at least 100 years, ie. Indianapolis Motor Speedway. Is this the same as the classic Driveway vs Parkway dichotomy?
    – user10126
    Sep 15, 2011 at 21:06
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    At first I agreed with this definition, but upon reflection it's incomplete and confusing. People don't refer to "motorboats" or "Motor speedways" because they're mixed up or the first vehicles were electric. It's true that engines rely on combustion, or more generally, heat conversion; but motors are not always electric. Otherwise "electric motor" would be a redundant phrase. An engine is a specific type of motor. See my answer.
    – ghoppe
    Sep 15, 2011 at 21:31
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    This is nonsense.
    – Robusto
    Sep 15, 2011 at 21:42
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    Here is an example of NASA referring to a rocket motor: clearly a combustion-based device ( nasa.gov/returntoflight/system/system_SRB.html )
    – horatio
    Sep 15, 2011 at 21:43
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    @Thursagen In my 25 years of software development I have seen lots of software that is combustible ;-)
    – user10126
    Sep 15, 2011 at 21:56