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macula
A dark spot.

magnetic
The property of a material to attract iron, cobalt, or nickel.

magnetosphere
The region of space in which a planet's magnetic field dominates that of the solar wind. The asymmetrically shaped volume around a magnetic body in which charged particles are subject to the bodies magnetic field rather than the magnetic field of another body such as the Sun or Jupiter.

magnetotail
The portion of a planetary magnetosphere which is pushed away from the Sun by the solar wind.

magnitude
The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Also called apparent magnitude.

mare
Literally "sea" (a very bad misnomer, still in use for historical reasons); really a large circular plain (plural: "maria").

Mars
Fourth planet from the sun. Named for the Roman god of war. Other than Earth, Mars is the most explored planet in our solar system.
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mass wasting
The downslope movement of rock, regolith, and/or soil under the influence of gravity.

mensa
A mesa. Flat-topped elevation.

Mercury
Closest planet to the sun. Named for the Roman god of trade a profit.
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metal
Used by astrophysicists to refer to all elements except hydrogen and helium, as in: "the universe is composed of hydrogen, helium and traces of metals". (Note: this is quite different from the usual chemistry definition.)

meteor
(also "shooting star" or "falling star") a bright streak of light in the sky caused by the entry into Earth's atmosphere of a meteoroid or a small icy particle. Very large, bright ones are called fireballs and bolides. (Photo credit: P. Spurny).

meteorite
A rock of extra-terrestrial origin found on Earth.

meteoroid
A small rocky object orbiting the Sun; smaller than an asteroid.

methane
A colorless and odorless gas that burns. It is often used as a fuel for heating homes and cooking food.

micron
A unit of distance equivalent to one-millionth (10E-6) of a meter; a micrometer. (E. coli, a common bacteria, is 0.5 microns in length.

mid-ocean ridge
A continuous mountain range with a central valley, located on the ocean floor where two tectonic plates move away from each other allowing molten rock from the Earth's interior to move toward the surface.

Milky Way Galaxy
The spiral galaxy containing our sun. Visible on the northern hemisphere in summer in the night sky as the Milky Way, a faint band stretching over the sky.

millibar
1/1000 of a bar. Standard sea-level pressure is about 1013 millibars.

minor planets
The official term used for asteroids.
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mons
A mountain. Plural: montes.

moon
A body which orbits a planet.

morphology
The scientific study of form, and of the structures and development that influence form. In geology, the external structure, form, and arrangement of rocks in relation to the development of landforms.