The sound of an orchestra tuning is instantly recognizable. As soon as you hear it, you know exactly what's about to happen. But there is a simple reason why a whole lot of tuning orchestras sound awfully alike - they are tuning to the same pitch frequency, an A at 440 hertz, abbreviated "Hz," a hertz is the unit by which we measure frequency.
But that makes you wonder - how did they choose that particular pitch? Was there some kind of mega-music conference where the powers that be decided that A440 was to be the tuning standard? Yes. Yes there was. Several, in fact.
The official 440 Hz standard, recognized by orchestras all over the world, was first declared the norm by the International Organization for Standardization in 1955, and they doubled down on that frequency 20 years later. ISO, the initials are derived from the French name of the organization, is dedicated to, as its name suggests, standardizing stuff. So, it was only a matter of time before they got around to standardizing sound itself.
Their decision was based off of the decision to declare 440 Hz right and proper at an early conference - that of a British Standards Institution meeting of 1939. And that frequency was a follow up to yet another conference, this one in Austria in 1885. The frequency decided there? The note A should be tuned to 440 Hz.
Tuning standards are the product of decades of meetings, which is probably the most boring answer you could get for practically any question.