In this Act, unless the context otherwise requires,-

(18). "input tax", in relation to a tax period on or after coming into force of this Act, means the amount of tax,-

(a) paid or payable under the Act, other than section 11, by a registered dealer, other than those enjoying composition under subsection (3), sub-section (3A), sub-section (3B) or sub-section (6), of section 16 or sub-section (4) of section 18, to a registered dealer, or a dealer who has made an application under sub-section (1) of section 24 within thirty days from the date of incurring liability to pay tax under the Act, at the time of purchasing taxable goods, other than such taxable goods as may be prescribed, during that period,