The word meteoroid is analogously formed from the word meteor, which means any atmospheric phenomenon and only in a more modern and narrower sense a phenomenon connected with the emission of light, and the ending -oid, which is taken from words like asteroid or planetoid. A meteoroid is defined as a celestial object, that is larger than dust, but smaller than an asteroid. There is no definition about the thresholds. There even isn’t any unofficial suggestion of a threshold. Meteoroids are in truth only called this way because they can cause meteors, but they won’t cause meteors unless they enter an atmosphere. So there’s no clear definition. Meteoroids can be smaller than 1 mm and in this case will they not burn (seen as meteors), but slowed down that much by the atmosphere that they slowly hover to the ground as dust particles. So meteoroids are actually dust because the definition includes this and fails in the attempt to exclude dust particles. Meteoroids originate from planets and asteroids, when these are hit by another object. So when two asteroids collide or a big (means not stopped by the atmosphere and of course depending on the atmosphere) meteoroid hits the surface of a planet, then a fountain of dust emerges from there and when the gravity can’t bring the dust back, then a stream of dust travels through the solar system. From such streams of dust come regular meteoroids and visit us during certain days of the year. They are expected and announced in the news and called shooting stars. Irregular meteoroids seldom hit the Earth because they are results of chain reactions and bring material from Mars or Moon to Earth and hence bring chances for new scientific insights. Bigger meteoroids leave a remnant, which is called meteorite. Some meteorites comprise metals with structures, which never form on Earth. It’s currently trendy to grind such meteorites and wear them like gemstones. Although a meteoroid is defined as not only bigger than a grain of dust, but can have a size in the magnitude of micrometers, but also as smaller as an asteroid, also this part of the definition isn’t binding and some meteoroids are also asteroids. Whether or not a meteoroid is also an asteroid depends only on whether it is arbitrarily found worthy to get numbered. What meteoroids aren’t doing is destroying cities or extinguish species. Such stories are all inventions from Hollywood. They were originally not expected to be widely believed!