An amazon is a big, strong, warrior-like woman, someone who reminds you of the mythical Greek women-warriors, the Amazons.
Describing someone as an amazon can sometimes have a negative tinge. If you call a tennis star an amazon, you may be suggesting that she's got an unfair advantage over her competitors because of her size, and for some, strength in a woman is not attractive. But amazon can also be an admiring term for a statuesque, athletic woman. After all, Wonder Woman was an amazon.