There are multiple definitions - click on the translation that matches your context.
capital gain means:
Substantiv
full
noun
Although the German phrase "Veräusserungsgewinn" literally means simply a "gain from a/the sale" and can therefore apply to a whole range of contexts, if the discussion is centered on the tax treatment of such a gain, then this phrase is the better translation.
profits made on the sale of property, an investment, or some other asset
The Democrats claim that tax cuts on dividends and long-term capital gains are simply a tax break for the rich.