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A planetary classification used to describe those planets, and especially extrasolar planets, with a mass substantially similar to that of Earth. This is one of three broad (and somewhat arbitrary) classifications used of terrestrial, rocky planets with a mass too low to be considered gas giants or ice giants. Such planets that are nonetheless more massive than Earth are classed as 'super Earths', while those far less massive are designated 'sub-Earths'. The term 'Earth-mass planet' is used for planets falling between these two more extreme types, typically with a mass close to or somewhat lower than that of Earth itself (though more massive planets are also sometimes included, up to several times the mass of Earth in some cases).
There are two Earth-mass planets within the Solar System. One of these is, of course, Earth itself, and Venus also falls into this category, with a mass 0.82 times that of Earth. On this system, Mercury and Mars would be classified as 'sub-Earths', as would the various dwarf planets throughout the system such as Ceres or Pluto.
Beyond the Solar System, Earth-mass planets appear to be relatively commonplace. The closest example would be Proxima Centauri b, orbiting the nearest star to the Sun, Proxima Centauri. It is not unusual to find stellar systems with several such planets, with a significant example being the red dwarf TRAPPIST-1, which has seven known planets, all of which fall into the 'Earth-mass planet' category.
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