The white lily is a flower familiar to most, both because of its beauty and because of the rich symbolic meaning various cultures have given to it.
The white lily's scientific name is Lilium candidum. According to its USDA PLANTS profile, it's a member of the lily family (Liliaceae) and genus (Lilium). It is a six-petaled flower with six stamens, white in colour with yellow at the base of each flower. Desert Tropicals states that it originates from the Balkans and Middle East.
The white lily is also commonly referred to as the Madonna Lily and is strongly associated with the purity of the Virgin Mary, especially in Roman Catholicism.
Meaning in the Song of Songs
Some scholars, including Abraham Bin Ezra, assert that the word "shoshannah" in the biblical Song of Songs refers to the white lily, and not to a rose as is commonly thought. Thus, the narrator describes his love's exceptional beauty by saying, "As the lily among thorns, so is my love among the daughters."
Meaning in the Victorian Era
The Victorians assigned detailed meanings to all common flowers and used them to communicate with each other. In her book "The Language of Flowers", Kate Greenaway listed the white lily as representing purity, modesty, and sweetness.
Because of the strong cultural link between the white lily and the qualities of purity and modesty, this flower pops up in figurative descriptions (sometimes including negative or sarcastic ones) of people as pure and innocent. "Lily-white" is the most common phrase used to describe people in this way.