That symbolical system which was developed by the early Greek or
Byzantine artists out of the Christian symbolism. Its chief
features are the circle, dome, and round arch; and its chief
symbols the lily, cross, vesica, and nimbus. St. Sophia, at
Constantinople, and St. Mark, at Venice, are excellent examples.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Byzantine Art from Infoplease: