Icon

In Brann's essay on Pictures in Poetry about Keats's "Ode on a Grecian Urn," he uses the word icon and iconic mulitple times.
An icon is a type of sighn that signifies what is represents by its inherent similiarity to that object, person, or place.

In Keats's poem the urn could be iconic of reality or eternity.
However, his poem is placed in the tradition of iconic poetry were poetry is like a picture. The iconic poem refers and represents a prevailing idea.