A female tiger is called a tigress. The earliest use of the word dates from the first decade of the 1600s and appends a French suffix, "-ess," to the word tiger to denote a feminine individual of the species. The original spelling of the word was actually tigresse.
According to Dictionary.com, tigresse reflects the original French spelling of the suffix. Oddly enough, the Latin word for tiger, which is also its species name, is tigris, which in pronunciation and form is closer to tigress than the generic English tiger. Tigris as a word is derived from Greek and is related to the Tigris River's name in that both are from an ancient Persian root word meaning pointed.