For purposes of this subchapter, the term:
(1) “Corporation” shall, for taxable years beginning after December 31, 1980, include financial institutions.
(2) “Taxable income” means the amount of net income derived from sources within the District within the meaning of §§ 47-1810.01 to 47-1810.03.
(3) “Taxable period” means a taxable year or a portion of a taxable year.