What is Capital Gains Tax?

Legal Definition
A capital gains tax (CGT) is a tax on capital gains, the profit realized on the sale of a non-inventory asset that was purchased at a cost amount that was lower than the amount realized on the sale. The most common capital gains are realized from the sale of stocks, bonds, precious metals and property. Not all countries implement a capital gains tax and most have different rates of taxation for individuals and corporations.

For equities, an example of a popular and liquid asset, national and state legislation often has a large array of fiscal obligations that must be respected regarding capital gains. Taxes are charged by the state over the transactions, dividends and capital gains on the stock market. However, these fiscal obligations may vary from jurisdiction to jurisdiction.
-- Wikipedia
Legal Definition
When a fixed asset is sold at a profit, the profit may be liable to a tax called Capital Gains Tax. Calculating the tax can be a complicated affair (capital gains allowances, adjustments for inflation and different computations depending on the age of the asset are all considerations you will need to take on board).