degrade

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

• intransitive verb To lower in quality or value; make inferior or less valuable.
• intransitive verb To lower in dignity; dishonor or disgrace: synonym: debase.
• intransitive verb To reduce in grade, rank, or status; demote.
• intransitive verb Geology To lower or wear away by erosion or weathering.
• intransitive verb To cause (an organic compound) to undergo degradation.
• intransitive verb To fall to a lower rank or status.
• intransitive verb To undergo degradation; decompose.

from The Century Dictionary.

• To reduce from a higher to a lower rank, degree, or type. Specifically
• To deprive of any office or dignity; strip of honors: as, to degrade a general officer.
• To lower in character; cause to deteriorate; lessen the value or worth of; debase: as, drunkenness degrades a man to the level of a beast.
• In biology: To reduce in taxonomic rank; lower in the scale of classification: as, to degrade an order to the rank of a family.
• To reduce in complexity of structure or function; simplify morphologically or physiologically: as, an organism degraded by parasitic habit.
• In geology, to reduce in altitude or magnitude, as hills and mountains or icebergs; wear down, as by the weather.
• In optics, to lower in position in the spectrum; increase the wave-length of (a ray of light), and hence diminish (its) refrangibility, as by the action of a fluorescent substance. See fluorescence.
• To diminish the strength, purity, size, etc., of.
• Synonyms and Debase, Disgrace, etc. (see abase); to dishonor, break, cashier, reduce to inferior rank. To lower, sink, impair, injure, pervert, pollute. See list under debase.
• In natural history, to degenerate in type; pass from a higher type of structure to a lower.
• To degenerate; become lower in character; deteriorate.
• In a university, to take, for some particular reason, a lower degree than one is entitled to, or to avoid taking a degree at the proper or usual time; descend from a higher to a lower degree.
• In thermodynamics, to convert from a form of greater to one of less availability: said of certain transformations of energy.

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

• intransitive verb (Biol.) To degenerate; to pass from a higher to a lower type of structure.
• transitive verb To reduce from a higher to a lower rank or degree; to lower in rank; to deprive of office or dignity; to strip of honors.
• transitive verb To reduce in estimation, character, or reputation; to lessen the value of; to lower the physical, moral, or intellectual character of; to debase; to bring shame or contempt upon; to disgrace.
• transitive verb (Geol.) To reduce in altitude or magnitude, as hills and mountains; to wear down.

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

• verb transitive To lower in value or social position.
• verb intransitive To reduce in quality or purity.

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

• verb lower the grade of something; reduce its worth
• verb reduce the level of land, as by erosion
• verb reduce in worth or character, usually verbally