How it differs from income gain
As opposed to dividends and distributions earned while continuing to hold an asset (income gain), capital gain is a profit that is realized at the moment an asset is sold. Conversely, a loss realized by selling at a lower price than the purchase price is called a "capital loss."
What to keep in mind
Because capital gain involves price-fluctuation risk, it is not guaranteed. Gains on sale are, in principle, subject to tax, and in some cases can be offset against losses from selling other stocks. It is also worth keeping in mind that past price movements do not guarantee future gains.