A biblical allusion is a literary device that makes an indirect reference to the bible in a written work. An allusion can refer to a person, idea, place or thing. An example of a biblical allusion could be "the city had so much crime that it was a modern-day Gomorrah." The biblical allusion, here, is the biblical city of Gomorrah, where crime and sin flourished. Dante's "Inferno" is a great example of a work that has many biblical allusions.
The idea of an allusion is to be subtle and to draw upon an assumption that the reader has prior knowledge of this outside work. In Western literature, there is often the assumption that the reader has at least some knowledge of the Bible, which is seen in its frequent use of biblical allusions. Indeed, many readers of Western literature who were not originally familiar with the Bible will ultimately learn some of the Bible's most popular stories through biblical allusions in Western literature.
Another popular example of a biblical allusion that is still popular is the term "good Samaritan." Applied to a person who helps another without knowing them, it is an allusion to the biblical parable, found in the story of Luke.