WordNet sense 1 (a possibility due to a favorable combination of circumstances):

opportunity

WordNet sense 2 (the possibility of future success):

prospect, prospect

WordNet sense 3 (an unknown and unpredictable phenomenon that causes an event to result one way rather than another):

fortune, hazard, luck

WordNet sense 4 (a measure of how likely it is that some event will occur; a number expressing the ratio of favorable cases to the whole number of cases possible):

probability

WordNet sense 5 (take a risk in the hope of a favorable outcome):

adventure, gamble, risk, run a risk, take a chance, take chances

WordNet sense 6 (come upon, as if by accident; meet with):

bump, find, happen, encounter

WordNet sense 7 (occurring or appearing or singled out by chance):

accidental, casual

WordNet sense 8 (without apparent forethought or prompting or planning):

unplanned