A terrestrial planet is mostly made of rock (silicate) Earth is the "original" terrestrial planet. When astronomers started understanding the kinds of planet they extended the term to include our nearest rocky neighbours: Mercury, Venus and Mars.
It is often said that they are similar to Earth. This is true of most of the structure and composition, but not the surface or the atmosphere. A terrestrial planet may be much hotter or colder than Earth and may, or may not, have much atmosphere.
With the discovery of planets orbiting other stars (exoplanets), the term terrestrial planet has been extended again to any rocky (silicate) planet orbiting any star.
All have a core, a mantle, and a crust. They are a bit like a boiled egg: the central yolk is the core; the white albumin is the mantle; and the shell is the crust. The crust of a terrestrial planet is thin, with the core and the mantle taking up the vast bulk, sometimes with a very large core, sometimes much smaller. Terrestrial planets have metallic cores of mostly iron, with rocky mantles and crusts.
The Moon is similar, but has a much smaller iron core. Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. Terrestrial planets can have canyons, craters, mountains, volcanoes, and other surface structures, depending on the presence of water and tectonic activity. Terrestrial planets have secondary atmospheres, got from volcanism, meteorites, and photosynthesis. In the giant planets, their atmospheres are primary, captured directly from the original solar nebula.
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Terrestrial planet Facts for Kids. Kiddle Encyclopedia.