As the other answers said, the standard for pitch was different in different locations and at different historical times, and even in a single place and time different standards were used for different types of music (e.g. secular and religious music).
Most wind and brass instruments have a narrow range of pitch adjustment, because the pitch is defined by the physical size of the instrument. Since manufacturing reproduction instruments for performing "early music" is now an international "industry", there is a general agreement on A = 415 Hz as a standard pitch, at least for the baroque period. That is close to a semitone below the standard pitch of A = 440, in which G sharp is 415.3Hz in equal-tempered tuning.
This choice of a semitone difference between the two "standards" means that instruments which can't easily be retuned (e.g. pipe organs) can be used at either pitch - either by the performer transposing the music by a semitone, or by a mechanical transposing device.
The difference of only a semitone might seem fairly immaterial, but it is significant in vocal music, particularly in soprano parts where the top note is often a written A. Lowering the pitch of such parts by a semitone can make a big difference in choirs made up of non-professional singers, where the "comfort zone" for the sopranos is more likely to stop at top G, not top A.