Definition from Wikipedia..
A440 is the musical note A above middle C. It has a frequency of 440 Hz and serves as a general tuning standard for musical pitch.

Prior to the standardization on 440 Hz, many countries and organizations followed the Austrian government's 1885 recommendation of 435 Hz. The American music industry reached an informal standard of 440 Hz in 1926, and used it in instrument manufacturing. In 1936 the American Standards Association recommended that the A above middle C be tuned to 440 Hz.[1] This standard was taken up by the International Organization for Standardization in 1955 (reaffirmed by them in 1975) as ISO 16.[2] Although still not universally accepted, since then it has served as the audio frequency reference for the calibration of acoustic equipment and the tuning of pianos, violins, and other musical instruments.

In the period instrument movement, a consensus has arisen around a modern "baroque pitch" for strings (Kammerton) of 415 Hz, baroque for wind instruments and church voices (Chorton) at 466 Hz and "classical pitch" at 430 Hz
BTW those clip on tuners are very useful for the practice shed. As they show you where you are all the time, and allow the player to adjust. It is also useful for long tones in a band, so you can check your own tuning and ensure it is consistent.
I still prefer the KORG TM-40 as it does show where the band is as well.