Symbol: an image associated by previous experience with a specific significance. Sometimes, symbolic images are augmented by words to insure that they are correctly identified.
E.g., The protagonist of William Faulkner's Light in August is named "Joe Christmas." He is associated with innocence, suffers for others' bad behavior, is mutilated and killed in a striking fashion by a bad man, and later appears in a vision to a man seeking understanding. Most readers consider him a "Christ symbol" because of the numerous functional similarities between his life and that of Jesus, an interpretation of the narrative they believe to be warranted by Faulkner's choice of his name, which alludes to the holiday celebrating Jesus' birth and gives the character the same initials as "Jesus Christ." Analysis of the novel that took Joe Christmas's symbolic element into account would have to consider both the similarities and the differences between the character and the secondary significance with which he was symbolically freighted. That is, even ordinary competent readers can be aware of the symbolic reference, so the scholar's work would necessarily have to look for non-obvious significance, perhaps in the ways in which Faulkner's "JC" differs from the biblical one.
Actions also can be symbolic, as when Fitzgerald's Gatsby stares longingly at the green light burning in the night at the end of Daisy's dock, perhaps suggesting a worshiper who seeks a sign of consent from the goddess he worships. In this case the symbolic reference is less tightly controlled by the author, resulting in a text Roland Barthes might call more "readerly," more susceptible to manipulation by its audience. For instance, the green light on a channel marker in U.S. coastal waters marks the left side of the channel for boats entering it (is Daisy a radical?), as opposed to red lights, which mark the right side of the channel for entering boats. What do you suppose Fitzgerald might mean by making it unclear whether Gatsby is "entering Daisy's channel" or not? Ambiguous symbols often can be as useful as those whose significances are tightly controlled by the authors.