The term Geosphere is often used to refer to the densest parts of Earth, which consist mostly of rock and regolith. The term originally applies to the four nested geospheres identified since Meteorology (Aristotle) with the states of terrestrial matter: solid (earth), liquid (water), gas (air), and plasma (fire). The nested geospheres then include the asthenosphere, lithosphere, hydrosphere, atmosphere, and the ionosphere or plasmasphere. The dense geosphere is also subdivided into the crust, mantle, and core. The outer core is unusual in that it is considered to be a liquid, yet it is a part of Earth's interior. In modern texts, geosphere refers to the solid parts of the Earth and is used along with atmosphere, hydrosphere, and biosphere to describe the systems of the Earth. In that context, some geologists prefer "lithosphere" over geosphere, but these can be used interchangeably. Since space exploration began, it has been observed that the extent of the ionosphere or plasmasphere is highly variable, and often much larger than previously appreciated, at times extending to the boundaries of the Earth's magnetosphere or geomagnetosphere. This highly variable outer boundary of geogenic matter has been referred to as the "geopause", to suggest the relative scarcity of geogenic matter beyond it, where the solar wind dominates.