There seems to be a fair amount of confusion surrounding the meaning of the prefix bi when used with units of time measurement. Biweekly, according to dictionary.com, can mean either "occurring twice weekly," or "occurring once every two weeks." Falling within the same realm of confusion are bimonthly and biannually.
As of late, it seems that we've begun replacing the bi prefix with semi when the intended meaning is "occurring twice within," while continuing to use the former when the intended meaning is "occurring once every other." Though this is a step in the right direction, there still seems to be confusion around proper usage.
Whose genius idea was it to start using the bi prefix to mean two very different things? If my employer told me my pay periods were bimonthly, should I assume they meant semimonthly or once every other month?
twice an x, or asonce every two x?". This could give helpful answers to potential visitors from the internets. – Cerberus Mar 10 '11 at 0:53