As used in the regulations in this part, unless otherwise expressly indicated:

(a) The terms defined in the provisions of law contained in the regulations in this part shall have the meanings so assigned to them.

(b) The Internal Revenue Code of 1954 means the Act approved August 16, 1954 (68A Stat.), entitled “An Act to revise the internal revenue laws of the United States”, as amended.

(c) District director means district director of internal revenue. The term also includes the Director of International Operations in all cases where the authority to perform the functions which may be performed by a district director has been delegated to the Director of International Operations.

(d) Calendar quarter means a period of 3 calendar months ending on March 31, June 30, September 30, or December 31.