The meaning of improvement used in section 48 of the Income Tax Act is not defined in the act itself. But we can take guidance from decisions by various high courts. Here is an excerpt of the court’s order in which the Hon’ble High Court tried to explain the meaning of improvement.
Meaning of improvement defined by court
The phrase “improvement” used in section 48 of the Income Tax Act came up before hon’ble Madras High Court in CIT Vs. V. Ramaswamy Mudaliar  196 ITR 939 (MAD.) and it explained
Meaning : The word ‘improve’ has various shades of meaning and it includes everything by doing which there is an enhancement in the value of the asset or there is a rise in its price or the asset is made to grow better or it is even followed up by something better.
Section 48 of Income Tax Act
Mode of computation.
48. The income chargeable under the head “Capital gains” shall be computed, by deducting from the full value of the consideration received or accruing as a result of the transfer of the capital asset the following amounts, namely :— (i) expenditure incurred wholly and exclusively in connection with such transfer; (ii) the cost of acquisition of the asset and the cost of any improvement thereto;
[(iii) in case of value of any money or capital asset received by a specified person from a specified entity referred to in subsection (4) of section 45, the amount chargeable to income-tax as income of such specified entity under that sub-section which is attributable to the capital asset being transferred by the specified entity, calculated in the prescribed manner:]
Provided that in the case of an assessee, who is a non-resident, capital gains arising from the transfer of a capital asset being shares in, or debentures of, an Indian company shall be computed by converting the cost of acquisition, expenditure incurred wholly and exclusively in connection with such transfer and the full value of the consideration received or accruing as a result of the transfer of the capital asset into the same foreign currency as was initially utilised in the purchase of the shares or debentures, and the capital gains so computed in such foreign currency shall be reconverted into Indian currency, so, however,
that the aforesaid manner of computation of capital gains shall be applicable in respect of capital gains accruing or arising from every reinvestment thereafter in, and sale of, shares in, or debentures of, an Indian company :
Provided further that where long-term capital gain arises from the transfer of a long-term capital asset, other than capital gain arising to a non-resident from the transfer of shares in, or debentures of, an Indian company referred to in the first proviso, the provisions of clause (ii) shall have effect as if for the words “cost of acquisition” and “cost of any improvement”, the words “indexed cost of acquisition” and “indexed cost of any improvement” had respectively been substituted:
Provided also that nothing contained in the first and second provisos shall apply to the capital gains arising from the transfer of a long-term capital asset being an equity share in a company or a unit of an equity oriented fund or a unit of a business trust referred to in section 112A:
Provided also that nothing contained in the second proviso shall apply to the long-term capital gain arising from the transfer of a long-term capital asset, being a bond or debenture other than—
(a) capital indexed bonds issued by the Government; or
(b) Sovereign Gold Bond issued by the Reserve Bank of India under the Sovereign Gold Bond Scheme, 2015:
Provided also that in case of an assessee being a non-resident, any gains arising on account of appreciation of rupee against a foreign currency at the time of redemption of rupee denominated bond of an Indian company held by him, shall be ignored for the purposes of computation of full value of consideration under this section:
Provided also that where shares, debentures or warrants referred to in the proviso to clause (iii) of section 47 are transferred under a gift or an irrevocable trust, the market value on the date of such transfer shall be deemed to be the full value of consideration received or accruing as a result of transfer for the purposes of this section :
Provided also that no deduction shall be allowed in computing the income chargeable under the head “Capital gains” in respect of any sum paid on account of securities transaction tax under Chapter VII of the Finance (No. 2) Act, 2004.
Explanation.—For the purposes of this section,—
(i) “foreign currency” and “Indian currency” shall have the meanings respectively assigned to them in section 2 of the Foreign Exchange Management Act, 1999 (42 of 1999);
(ii) the conversion of Indian currency into foreign currency and the reconversion of foreign currency into Indian currency shall be at the rate of exchange prescribed in this behalf;
(iii) “indexed cost of acquisition” means an amount which bears to the cost of acquisition the same proportion as Cost Inflation Index for the year in which the asset is transferred bears to the Cost Inflation Index for the first year in which the asset was held by the assessee or for the year beginning on the 1st day of April, 2001, whichever is later;
(iv) “indexed cost of any improvement” means an amount which bears to the cost of improvement the same proportion as Cost Inflation Index for the year in which the asset is transferred bears to the Cost Inflation Index for the year in which the improvement to the asset took place;
(v) “Cost Inflation Index”, in relation to a previous year, means such Index as the Central Government may, having regard to seventy-five per cent of average rise in the Consumer Price Index (urban) for the immediately preceding previous year to such previous year, by notification31 in the Official Gazette, specify, in this behalf.
Updated up to Finance Act 2021.