discord

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

• noun Lack of agreement among persons, groups, or things. synonym: conflict.
• noun Tension or strife resulting from a lack of agreement; dissension.
• noun A confused or harsh sound or mingling of sounds.
• noun Music An inharmonious combination of simultaneously sounded tones; a dissonance.
• intransitive verb To fail to agree or harmonize; clash.

from The Century Dictionary.

• To disagree; jar; clash.
• To be discordant or dissonant.
• noun Want of concord or harmony between persons or things; disagreement of relations; especially, as applied to persons, difference of opinions; variance; opposition; contention; strife; any disagreement which produces passion, contest, disputes, litigation, or war.
• noun In music: The combination of two tones that are inharmonious with each other, or inconclusive in combined effect; a dissonance.
• noun The interval between two such tones; any interval not a unison, octave, perfect fifth, perfect fourth, major or minor third, or major or minor sixth. In medieval music all but the first three of the above intervals were at first regarded as discords.
• noun Either of the two tones forming such an interval.
• noun A chord containing such intervals.See dissonance.
• noun Hence—3. Any confused noise; a mingling or clashing of sounds; a harsh clang or uproar.

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

• noun Want of concord or agreement; absence of unity or harmony in sentiment or action; variance leading to contention and strife; disagreement; -- applied to persons or to things, and to thoughts, feelings, or purposes.
• noun (Mus.) Union of musical sounds which strikes the ear harshly or disagreeably, owing to the incommensurability of the vibrations which they produce; want of musical concord or harmony; a chord demanding resolution into a concord.
• noun See under Apple.
• intransitive verb obsolete To disagree; to be discordant; to jar; to clash; not to suit.

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

• noun Lack of concord, agreement or harmony among persons, groups, or things.
• noun Tension or strife resulting from a lack of agreement; dissension.
• noun music An inharmonious combination of simultaneously sounded tones; a dissonance.
• noun Any harsh noise, or confused mingling of sounds.
• verb archaic To disagree; to be at variance; to fail to agree or harmonize; clash.

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

• verb be different from one another
• noun a harsh mixture of sounds
• noun strife resulting from a lack of agreement
• noun lack of agreement or harmony
• noun disagreement among those expected to cooperate