Besiyata Dishmaya
Besiyata Dishmaya (Aramaic: בסיעתא דשמיא) is an Aramaic phrase, meaning "with the help of Heaven". The acronym BS"D (Mostly written in Hebrew: בס"ד) has become a Jewish term amongst several orthodox religious denominations, reproduced at the top of every written document (beginnings of correspondences, letters, notes, etc.) as a reminder to them that all comes from God, including the following content and to contextualize what's really important in the text, that without God's help we can do nothing successfully. BS"D is not derived from any religious law of the Halakha but it's considered an old accepted tradition.
Acronym
The reason for the common use of the three-letter acronym, בס"ד (BS"D), is probably because it does not contain the Hebrew letter Hei (ה), that is used to imply the name of God, and for this reason, a page which contains the letters בס"ד (BS"D) without any other Torah content does not require Genizah (a process for writings that contain the name of God), and thus can be thrown away without a fear of violation.
