Inclination;Angle between the orbit plane of a planet and the plane of the ecliptic.
Infrared radiation;Electromagnetic radiation, located in the spectrum between the end of the long-wave spectrum and radio waves.
Infrared radiation;Low-energy electromagnetic radiation, with less energy than visible light.
Interplanetary matter;Matter within the solar system consisting of the minute particles of the solar wind, meteorite dust and gas molecules from atmospheres of planets.
Interstellar dust;Small, solid particles that are thinly dispersed between the stars in space.
Interstellar gas;Gases, mainly hydrogen and traces of helium and other elements, that are found between the stars in space.
Ion;Atom lacking an electron, or with an additional electron.
