The full form of BSW is: 

Explanation:

British Standard Whitworth (BSW) thread:

• It has an included angle of 55° and the depth of the thread is 0.6403 x P, where P = pitch.
• The crest and root are rounded off to a definite radius.
• The Figure below shows the relationship between the pitch and the other elements of the thread.
• BSW thread is represented in a drawing by giving the major diameter.
• For example: 1/2” BSW, 1/4” BSW.
• BSW thread is used for general-purpose fastening threads.