M:

Magnitude: numbers assigned to heavenly bodies indicating their relative brightness: the smaller the number, the brighter the object. Thus the brightest star, Sirius, with a magnitude of -1.44, is brighter than a star of magnitude 0, which in turn is brighter than a star of magnitude 1, etc. The faintest object the human eye can see is magnitude 6, about the brightness of the planet Uranus. The Hubble Space Telescope can see some faint galaxies as dim as the 30th magnitude, 4 billion times fainter than the faintest object the human eye can see. In general, objects are 2.512 times brighter than objects one magnitude less. Thus, a first magnitude star is 2.5x2.5.x2.5x2.5x2.5 = 100 times brighter than a star of magnitude 6. Venus, the brightest object in the sky other than the Sun or Moon, has a magnitude of -4.4. The full Moon’s magnitude is -12.7, and the Sun’s magnitude is -26.75.

Major Aspect: the conjunction, opposition, trine, square and sextile aspects; those aspects having angular separations of 360° divided by 1, 2, 3, 4 or 6, that is, with harmonic numbers of 1, 2, 3, 4 or 6.

Malefic(s): (now archaic) Mars and Saturn.

Masculine (Yang): assertive fire or air qualities; Aries, Gemini, Leo, Libra, Sagittarius, Aquarius.

Mass: An object's weight (not its mass) is a measure of how much the planet Earth pulls on that object.  Hold an object in your hand.  You can feel the Earth pulling it. But that feel of weight is a result of an interaction between the Earth and the object.  Newton established that all bodies in the universe attract each other with a force that increases as mass of the bodies increases.  Thus an object's weight depends on two things: how much stuff is in the object itself, and much stuff is in the object pulling on it, in this case, the Earth.  It follows that an object's weight is greater in the gravitational field of the Sun, and less in the gravitational field of the moon.

Newton realized that all objects have an intrinsic property that doesn't depend on the planet pulling on the object, a property that stays the same whether or not the object is on the Sun, the Moon, or on the Earth.  This intrinsic property of any body, independent of where it is in the universe, is called the body's mass. And a body's mass can be obtained by taking its weight -- which depends on how much a particular "planet" is pulling on it to make it "heavy" -- and dividing its weight by the strength of the particular planet's pull on it.

Now Newton discovered that the Earth pulls on (or accelerates) all objects with the same amount of force.  This is the gist of his apocryphal falling apple experience. If you were to drop 1 pound of feathers and a 1-pound lead ball from the same height in a vacuum, they would both hit the ground at the same time. Why "in a vacuum?" Because air resistance would impede the falling feathers more than the falling lead ball -- but they are both being pulled on (or accelerated by) the Earth with one pound of force, which is experienced on the Earth as their weight.

 Since the Earth's pull is a constant for all bodies, it can be easily calculated.  It is 32 feet/9.8 meters per second per second. The "per second per second" is because acceleration is a change in velocity, so the Earth's pull increases a falling body's velocity 32 feet per second every second. Thus the Earth's constant pull on all objects can be divided out of the weight of any object. And when you divide an object's weight by the pull of the body in whose gravitational field you're measuring its weight (like the Earth's pull), you're left with its "mass" (the units of which are "slugs" in the English system).

Thus a body's mass is independent of the planet in whose gravitational field you're weighing it. So although its weight differs on Mercury, the Sun, the Moon and Jupiter, its mass remains constant on all those bodies.  Mass is the amount of stuff a body has in it, not how hard a particular body or planet is pulling on that stuff -- which is its weight.

MC: "medium coeli," Latin for "middle of the sky"; the intersection of the local meridian with the ecliptic; the Midheaven, or tenth house cusp.

Meridian: any circle with its center at the Earth’s center passing through the observer’s zenith and the North or South Pole. The meridian is that "vertical circle" passing through the center of the Earth and perpendicular to the horizon (the definition of a vertical circle) that also passes through the north and south points on the horizon. The Meridian is also perpendicular to the Prime Vertical, the vertical circle passing through the east and west points on the horizon. The horizon and Meridian planes quarter the local sky and form the 12 houses when trisected by planes perpendicular to the Prime Vertical and passing through the horizon’s north - south line (see diagram on page 117). All local meridians intersect the ecliptic at the tenth and fourth house cusps. The 24 Standard Time Meridians are each 15° apart, and define the time within the irregularly shaped Time Zones around each of them.

Midheaven: The midheaven is the ecliptic degree most directly overhead: the intersection of the local meridian with the ecliptic above the horizon. The highest point in the local sky on the planet’s path, it is also known as the tenth house cusp, or the medium coeli (MC). It is one’s point of maximum externalization: the social world most distant from intimate, personal life. It is what we need to do with our life in order to grow.

Minor Aspect: the semisquare, quincunx, semisextile, sesquiquadrate, quintile, biquintile, septile, biseptile, triseptile, novile and decile.

Minor house(s): the second, third, fifth, sixth, eighth, ninth, eleventh and twelfth houses; any house other than the angular houses.

Minute (‘): the 60 equal divisions of a degree of arc. In a chart, the digits before the little sign symbol accompany the planetary glyphs show the position of each planet in the sign by degrees. The digits after the little sign symbol refine that position to 60ths of a degree, or minutes of arc. The Moon and Sun both subtend an arc of 30' in the sky -- which is why they appear to be the same size from the Earth and can exactly eclipse each other. Venus has an angular diameter of 1' at closest approach, and unaided, the human eye can resolve objects in detail about 1' across. 

Mutual Reception: the condition where two planets are each located in a sign ruled by the other. This is a beneficial condition, lending harmony and stability to each, and compensates for the detrimental signification otherwise occurring if either or both planets are in its detriment or fall.

Mutable: the third and last sign in each season, associated with flexibility and versatility. Gemini, Virgo, Sagittarius, and Pisces. A lot of Mutable energy in a chart indicates suggestibility, flexibility, and potentially a lack of stability.

Mystic Rectangle: two pairs of oppositions whose ends are trine and sextile to each other.