The frequency of the middle A note on a piano (that's the one above, i.e. further to the right, of middle C).
This hasn't always been the case, and there are still, in fact, exceptions, most notably in Baroque music. Through the years the frequency of A has ranged from about 390 Hertz to 460 Hz. A higher A is sometimes used in an attempt to give the instruments a "brighter" sound. Tuning an instrument higher can have the adverse effect of causing more broken strings, especially in the days when they were made of gut.
The next higher A would have a frequency of 880, then 1760, and so on.