| igneous rock | Any rock produced by cooling from a molten state. |
| inclination (of an orbit) | The angle between the orbital plane of a revolving body and some fundamental plane-usually the plane of the celestial equator or of the ecliptic. |
| inertia | The property of matter that requires a force to act on it to change its state of motion; the tendency of objects to continue doing what they are doing in the absence of outside forces. |
| inertial system | A system of coordinates that is not itself accelerated, but that either is at rest or is moving with constant velocity. |
| inflationary universe | A theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion during the first 10^-30 S. After this period of rapid expansion, the Big Bang and inflationary rdodels are identical. |
| infrared cirrus | Patches of interstellar dust, which emit infrared radiation and look like cirrus clouds on the images of the sky produced by the Infrared Astronomy Satellite. |
| infrared radiation | Electromagnetic radiation of wavelength longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths. |
| interstellar dust | Tiny solid grains in interstellar space, thought to consist of a core of rock-like material (silicates) or graphite surrounded by a mantle of ices. Water, methane, and ammonia are probably the most abundant ices. |
| interstellar extinction | The attenuation or absorption of light by dust in the interstellar medium. |
| inverse-square law (for light) | The amount of energy (light) flowing through a given area in a given time (flux) decreases in proportion to the square of the distance from the source of energy or light. |
| ion | An atom that has become electrically charged by the addition or loss of one or more electrons. |
| ionization | The process by which an atom gains or loses electrons. |
| ionosphere | The upper region of the Earth's atmosphere in which many of the atoms are ionized. |
| ion tail (of comet) | See plasma. |
| irregular galaxy | A galaxy without rotational symmetry; neither a spiral nor an elliptical galaxy. |
| isotropic | The same in all directions. |