(a)
In general
For purposes of this title in the case of income, estate, and gift taxes imposed by subtitles A and B and excise taxes imposed by chapters 41, 42, 43, and 44 the term “deficiency” means the amount by which the tax imposed by subtitle A or B, or chapter 41, 42, 43, or 44 exceeds the excess of—
(1)
the sum of
(A)
the amount shown as the tax by the taxpayer upon his return, if a return was made by the taxpayer and an amount was shown as the tax by the taxpayer thereon, plus
(B)
the amounts previously assessed (or collected without assessment) as a deficiency, over—
(2)
the amount of rebates, as defined in subsection (b)(2), made.