Black hole: A gravitationally collapsed mass inside the Schwarzschild radius (the critical radius, according to the general theory of relativity, at which light is unable to escape to infinity) from which no light, matter, or signal of any kind can escape; quantum corrections indicate a black hole radiates particles with a temperature inversely proportional to the mass and directly proportional to Planck's constant (the constant of proportionality relating the frequency of a photon to its quantum of energy).

Continental drift: The concept of continent formation by the fragmentation and movement of land masses on the surface of the Earth.

Electromagnetic waves: Disturbances propagating outward from any electric charge that oscillates or is accelerated; far from the charge, they consist of vibrating electric and magnetic fields that move at the speed of light and are at right angles to each other and to the direction of motion.

Gravitational waves: Also known as gravitational radiation; according to general relativity, a propagating gravitational field, which is produced by some change in the distribution of matter; generally, any body which experiences varying acceleration will emit gravitational waves by an amount proportional to the rate of change of the acceleration.

Jovian: Of, relating to, or characteristic of the planet Jupiter.

Magnetosphere: The region of space surrounding a rotating magnetized sphere. Specifically, the outer region of a planets ionosphere. On Earth, the magnetosphere starts at about 100 km (about 60 miles) above Earths surface, and extends to about 60,000 km (or considerably farther, on the side away from the Sun), i.e., to a distant boundary that marks the beginning of interplanetary space.

Maxwell's equations: Equations governing the varying electric and magnetic fields in a medium.

Microarcseconds: 1/1000th of a second of arc; a second of arc being a unit of plane angle equal to 1/60th of a minute; a minute being 1/60th of a degree.

Plasma: 1. A highly ionized gas that contains equal numbers of ions and electrons in sufficient density so that the Debye shielding length (the characteristic distance beyond which the electric field of a charged particle is shielded by particles having charges of the opposite sign. 2. A completely ionized gas composed entirely of a nearly equal number of positive and negative free charges (positive ions and electrons).

Quasar: Quasi-stellar astronomical object, often a radio source; all quasars have large red shifts and small optical diameter, but may have large radio diameter. Also known as quasi-stellar object (QSO).

Space-time: A four-dimensional space used to represent the Universe in the theory of relativity, with three dimensions corresponding to ordinary space and the fourth to time. Also known as space-time continuum.