Section 1. The following words as used in this chapter shall, unless the context otherwise requires, have the following meanings:—

''Corporation'', every corporation, company or association subject to taxation under the provisions of chapter sixty-three, or under any special act which is in lieu of the provisions of said chapter sixty-three.

''Estimated tax'', the amount which the corporation estimates as the taxes required to be paid under chapter sixty-three or any act in lieu thereof, and under any act in addition thereto.