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What is meant by "plant" in control system? Does it also means systems like DC motor or it only includes large systems like chemical (fertilizer or cement) plant?

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Indeed, as Marko writes, it can be just a DC motor. In general it's "the thing being controlled".

In fact the word "plant" in plain English can mean

  • "factory", as the OP points out (Cement factory)

and closer to the field of control theory it can also mean

  • "machines", without the factory building

For instance "the heating plant for a home" or "nuclear plant for a submarine"; https://www.dictionary.com/browse/plant

I'll let the linguists battle it out, but it does origin from the word for "shrub", which rooted in soil, with branches, leaves etc.. and if you look at a power plant or heating plant, they look like that too: mounted to the ground, with protruding tubes, boxes etc.... so maybe plant -the machine- came before plant -the factory.

See also the comment by Ross, who found the etymology back to the 18th century: "at first with reference to the machinery, tools, apparatus, etc., later also the building"

There's something that connects the non-mobile machine, installed at a factory, with feed pipes, exhaust pipes etc..., with the rooted, and thus obviously non-mobile, plant.

And according to Andrew (see comments), in British English it's quite common to refer to heavy and mobile machinery like bulldozers, graders, and road rollers as "heavy plant"

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Image: https://thoughtsbyanidlemind.wordpress.com/2013/02/15/thoughts-on-heavy-plant-crossing/

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    \$\begingroup\$ etymonline.com/search?q=plant ... Most extended usages are from the verb, on the notion of "something planted;" such as "construction for an industrial process," 1789, at first with reference to the machinery, tools, apparatus, etc., later also the building; also slang meaning "a spy" (1812). Many of these follow similar developments in the French form of the word. \$\endgroup\$
    – Ross Presser
    Sep 23, 2020 at 15:41
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    \$\begingroup\$ And there's also "heavy plant" such as bulldozers, graders, and road rollers, leading the slightly amusing road warning sign "Heavy plant crossing". \$\endgroup\$
    – Andrew Morton
    Sep 23, 2020 at 15:53
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    \$\begingroup\$ @P2000 sorry, no, I don't have an answer for that. \$\endgroup\$
    – Ross Presser
    Sep 23, 2020 at 17:04
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    \$\begingroup\$ Another example -- In HVAC the set of chillers or boilers / heat pumps and pumps, valves, control devices etc. is commonly referred to as a "plant". These devices are generally controlled as a system. \$\endgroup\$
    – StayOnTarget
    Sep 23, 2020 at 18:22
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    \$\begingroup\$ And most of the devices @UuDdLrLrSs refers to, along with others, are likely to be found in a "plant room". A lift (elevator) may also have its own plant room at the top of the shaft (or on the roof) \$\endgroup\$
    – Chris H
    Sep 23, 2020 at 18:54
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It could be anything that has an input and gives an output. In control theory is depicted as a box with a relation Output VS. Input, we call this transfer function. It may have also multiple inputs and/or multiple outputs.

The physical size is not important, so yes your DC motor could also be a plant.

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