A Mega-Earth, also known as a "massive solid planet", is a terrestrial planet which is at least 10 times the mass of Earth. Mega-Earths are substantially more massive than super-Earths (terrestrial and ocean planets with masses around 5-10 Earths). The term "mega-Earth" was coined in 2014, when Kepler-10c was revealed to be a Neptune-mass planet with a density considerably greater than that of Earth. However, it has since been determined to be a typical volatile-rich planet.
The first extrasolar planet that was classified as a "Mega-Earth" was Kepler-10c. At the time of its discovery, it was thought to have around 17 times the mass of Earth, but only 2.3 times the radius of Earth, giving it a high density that implied a mainly rocky composition. However, several follow-up radial velocity studies produced different results for Kepler-10c's mass, all much below the original 17 Earth mass estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around 7.4 Earth masses, making it a typical volatile-rich Mini-Neptune and not a Mega-Earth.
K2-56b, also known as BD+20594b, is a much more likely Mega-Earth, with about 16 Earth masses and 2.2 Earth radii. At the time of its discovery in 2016, it had the highest chance of being rocky for a planet its size, with a posterior probability that it is dense enough to be terrestrial at about 0.43. For comparison, at the time Kepler-10c had a Procky of 0.1, and Kepler-131b has a Procky of 0.002.
Kepler-145b is currently the most massive planet classified as a Mega-Earth, with a mass 37.1 times that of Earth, but only a radius of 2.65 Earths, which is so large that it may belong to a sub-category of Mega-Earth known as "Supermassive Terrestrial Planets" (SMTP). It likely has an Earth-like composition of rock and iron without any volatiles. A similar Mega-Earth, K2-66b, is about 21.3 times the mass and 2.49 times the radius of Earth, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.