1. Literally, the act of throwing a coin into the air, with the outcome of something dependent on which side of the coin lands face-up. We figured the fairest way was to do a coin flip to see who should keep Dad's old stamp collection.

2. By extension, a situation in which two outcomes, usually opposites, are equally likely and will be decided by chance. At this point, it's really just a coin flip whether we'll be able to recoup our costs from this project or not.