collision

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

• noun An act of colliding; a crash.
• noun A condition of opposition or conflict between two or more people or things.
• noun Physics A brief dynamic event consisting of the close approach of two or more particles, such as atoms, resulting in an abrupt change of momentum or exchange of energy.

from The Century Dictionary.

• To collide; strike against.
• noun The act of striking or dashing together; a striking together of two bodies; the meeting and mutual striking or clashing of two or more moving bodies, or of a moving body with a stationary one; specifically, in recent use, the dashing together of two railroad-trains, or of two boats or ships.
• noun Opposition; antagonism; counteraction: as, a collision of interests or of parties.
• noun See extract.
• noun Synonyms Concussion, etc. See shock.

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

• noun The act of striking together; a striking together, as of two hard bodies; a violent meeting, as of railroad trains; a clashing.
• noun A state of opposition; antagonism; interference.

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

• noun An instance of colliding.

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

• noun a conflict of opposed ideas or attitudes or goals
• noun (physics) a brief event in which two or more bodies come together
• noun an accident resulting from violent impact of a moving object