precession

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or state of preceding; precedence.
• noun Physics The motion of the axis of a spinning body, such as the wobble of a spinning top, when there is an external force acting on the axis.
• noun Precession of the equinoxes.
• noun A slow gyration of the earth's rotational axis around the pole of the ecliptic, caused by the gravitational pull of the sun, moon, and other planets on the earth's equatorial bulge.

from The Century Dictionary.

• noun In mech., the cyclic movement of the axis of rotation of a body which revolves about a free axis in a field of force. In order that precession shall occur the body must not be a homogeneous sphere and the axis of rotation must be inclined to the lines of force of the field.
• noun The act of going before or of moving forward; advance.
• noun Precedence.
• noun In philology, a weakening of a vowel due to a change of accent; a change from a full strong vowel to a thinner one: opposed to progression.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of going before, or forward.
• noun (Astron.) See under Lunisolar.
• noun that part of the precession of the equinoxes which depends on the action of the planets alone.
• noun (Astron.) the slow backward motion of the equinoctial points along the ecliptic, at the rate of 50.2″ annually, caused by the action of the sun, moon, and planets, upon the protuberant matter about the earth's equator, in connection with its diurnal rotation; -- so called because either equinox, owing to its westerly motion, comes to the meridian sooner each day than the point it would have occupied without the motion of precession, and thus precedes that point continually with reference to the time of transit and motion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable precedence
• noun physics, countable The wobbling motion of the axis of a spinning body when there is an external force acting on the axis.
• noun astronomy, uncountable The slow gyration of the earth’s axis around the pole of the ecliptic, caused mainly by the gravitational torque of the sun and moon.
• noun Any of several slow changes in an astronomical body's rotational or orbital parameters.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the motion of a spinning body (as a top) in which it wobbles so that the axis of rotation sweeps out a cone
• noun the act of preceding in time or order or rank (as in a ceremony)