chance

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

• noun The unknown and unpredictable element in happenings that seems to have no assignable cause.
• noun A force assumed to cause events that cannot be foreseen or controlled; luck.
• noun The likelihood of something happening; possibility or probability.
• noun An accidental or unpredictable event.
• noun A favorable set of circumstances; an opportunity.
• noun A risk or hazard; a gamble.
• noun Games A raffle or lottery ticket.
• noun Baseball An opportunity to make a putout or an assist that counts as an error if unsuccessful.
• adjective Caused by or ascribable to chance; unexpected, random, or casual.
• transitive verb To take the risk or hazard of.
• transitive verb To have the fortune (to be or do something); happen.
• transitive verb Used with the impersonal subject it and a following clause or infinitive to indicate the occurrence of a usually unexpected or chance event.
• idiom (by chance) Without plan; accidentally.
• idiom (by chance) Possibly; perchance.
• idiom (on the off chance) In the slight hope or possibility.

from The Century Dictionary.

• To happen; fall out; come or arrive without design or expectation.
• To befall or happen to.
• To risk; hazard; take the chances of: as, the thing may be dangerous, but I will chance it.
• By chance; perchance.
• noun Fall; falling.
• noun A throw of dice; the number turned up by a die.
• noun Hence Risk; hazard; a balanced possibility of gain or loss, particularly in gaming; uncertainty.
• noun A contingent or unexpected event; an event which might or might not befall.
• noun Vicissitude; contingent or unexpected events in a series or collectively.
• noun Luck; fortune; that which happens to or befalls one.
• noun Opportunity; a favorable contingency: as, now is your chance.
• noun Probability; the proportion of events favorable to a hypothesis out of all those which may occur: as, the chances are against your succeeding.
• noun Fortuity; especially, the absence of a cause necessitating an event, or the absence of any known reason why an event should turn out one way rather than another, spoken of as if it were a real agency; the variability of an event under given general conditions, viewed as a real agency.
• Resulting from or due to chance; casual; unexpected: as, a chance remark; a chance customer.
• Synonyms Casual, Fortuitous, etc. See accidental.

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

• transitive verb To take the chances of; to venture upon; -- usually with it as object.
• transitive verb obsolete To befall; to happen to.
• intransitive verb To happen, come, or arrive, without design or expectation.
• adjective Happening by chance; casual.
• noun A supposed material or psychical agent or mode of activity other than a force, law, or purpose; fortune; fate; -- in this sense often personified.
• noun The operation or activity of such agent.
• noun The supposed effect of such an agent; something that befalls, as the result of unknown or unconsidered forces; the issue of uncertain conditions; an event not calculated upon; an unexpected occurrence; a happening; accident; fortuity; casualty.
• noun A possibility; a likelihood; an opportunity; -- with reference to a doubtful result.
• noun (Math.) Probability.
• noun one who comes unexpectedly.
• noun the sole remaining ground of hope.
• noun the chief opportunity; that upon which reliance is had, esp. self-interest.
• noun (Math.) that branch of mathematics which treats of the probability of the occurrence of particular events, as the fall of dice in given positions.
• noun to take advantage of every circumstance; to seize every opportunity.
• adverb By chance; perchance.

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

• noun countable An opportunity or possibility.
• noun uncountable Random occurrence; luck.