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Wiktionary
capital gain

n. 1 (context economics English) An increase in the value of a capital asset 2 (context business finance English) An amount by which the value or the proceeds of the sale of a capital asset by its owner exceed its cost to the owner.

WordNet
capital gain

n. the amount by which the selling price of an asset exceeds the purchase price; the gain is realized when the asset is sold

Wikipedia
Capital gain

A capital gain is a profit that results from a sale of a capital asset, such as stock, bond or real estate, where the sale price exceeds the purchase price. The gain is the difference between a higher selling price and a lower purchase price. Conversely, a capital loss arises if the proceeds from the sale of a capital asset are less than the purchase price.

Capital gains may refer to "investment income" that arises in relation to real assets, such as property; financial assets, such as shares/stocks or bonds; and intangible assets.