Is that a trick question? Everyone knows what a plant is, right? They're everywhere: trees, flowers, weeds, crops. They're leafy green things. Couldn't be simpler.
Not quite.
In point of fact, plants need be neither green nor leafy. They need not be even visible to the naked eye. And lest you be tempted to get clever, they certainly don't need flowers, cones, seeds, or even spores.
Leaving the original question: what is a plant, then, smart-ass?
Plants comprise one of the four (five? pick a number!) kingdoms of life. Others include animals, of course, fungi, and various microscopic organisms (eg. Monera and Protista). Technically it has to do with chloroplasts, cell walls, cellulose, starch, and such like minutia. [1] As chlorophyll is green, and leaves are made of cellulose, this very loosely translates as "green and leafy", bringing us full circle.
A more useful definition simply enumerates plants as: green algae, mosses, ferns, and the much-celebrated and ever-lovely trees and flowers. By contrast, plants do not include mushrooms or lichens. And most definitely not sea cucumbers or couch potatoes, no matter how closely they may come to resemble their plant name-sakes.