What fruit is commonly associated with the biblical forbidden fruit?

Though the forbidden fruit in the Book of Genesis is not identified, popular Christian tradition holds that Adam and Eve ate an apple from the forbidden tree in the Garden of Eden. The unnamed fruit of Eden thus became an apple under the influence of the story of the golden apples in the Garden of Hesperides. As a result, the apple became a symbol of knowledge, immortality, temptation, the fall of man, and sin. According to the Bible, there is nothing to show the forbidden fruit of the Tree of Knowledge was necessarily an apple.

Source: Wikipedia