Type of fruit

In Western Europe, the fruit was depicted as an apple, because of a misunderstanding of, or perception of intentional dual meaning in, the Latin malus, which as an adjective means evil, but as a noun means apple. In the Vulgate, Genesis 2:17 describes the tree as "de ligno autem scientiae boni et mali" Genesis 2:17 ("mali" is the genitive of "malus"). The larynx in the human throat, noticeably more prominent in males, was consequently called an Adam's apple, from a notion that it was caused by the forbidden fruit sticking in Adam's throat as he swallowed, and the name has stuck. Some Slavonic texts state that the "forbidden fruit" was actually the grape, that was later changed in its nature and made into something good, much as the serpent was changed by losing its legs and speech. Other Eastern Christians sometimes assume that the "forbidden fruit" was the fig, from the account of their using leaves of this tree to cover themselves (also the fig tree is the only fruit tree explicitly mentioned in the Genesis 3 context). There are also opinions that it was a tomato (however, tomatoes are native to the American continent, thus unknown in Eurasia in biblical times) because this fruit in some Slavic languages is called "rajčica" or "paradajz", (both words are related to paradise - "raj" means "paradise"). Some Rabbinic and Islamic traditions regard the forbidden fruit as wheat : wheat is "khitah" in Hebrew and thus is a pun on khet, "sin" http://www.straightdope.com/columns/read/2682/was-the-forbidden-fruit-in-the-garden-of-eden-an-apple. Otherwise, Islam regards a fig or an olive as the forbidden fruit. Still, many believe the quince which pre-dates apples and native to Southeast Asia was the forbidden fruit.

In addition to the Rabbinic tradition concerning wheat, most of the other stated opinions also appear in Judaism besides the tomato which was not known during the Talmudic era, but it's most commonly referred to, as the citron or etrog.

In ancient Egypt and imperial China, various mushrooms that grew on trees were forbidden to eat because they were very rare and had desirable medicinal effects that were reserved for the Egyptian Pharaoh or Chinese Emperor. From this interpretation of forbidden fruit, many different types of trees could bear the forbidden mushroom fruit, and the fruit of the tree of life and fruit of the knowledge and good of evil are merely different categories of mushroom classified according to their effect, i.e. medicinal vs. psychotropic.

As a metaphor

The term most generally refers to any indulgence or pleasure that is considered illegal or immoral and potentially dangerous or harmful, particularly relating to human sexuality.