Final answer:
The profit earned by a bond trader from buying and then reselling a bond is called a capital gain, achieved when the resale price exceeds the purchase price.
Step-by-step explanation:
The profit a bond trader makes from purchasing and then reselling a bond is referred to as a capital gain. This gain occurs when the trader sells the bond for a higher price than the price they paid when buying it. The bond yield is the rate of return a bond is expected to pay at the time of purchase, and this is typically what attracts an investor to buy a bond. However, if the market conditions change in favor of the bond's value, the trader can sell the bond at a higher price than its face value or purchase price, hence realizing a capital gain.