The Great Gatsby, wafer of a moon

What is the meaning of "wafer" in this extract from The Great Gatsby by F. Scott Fitzgerald, chapter 3:

The caterwauling horns had reached a crescendo and I turned away and cut across the lawn toward home. I glanced back once. A wafer of a moon was shining over Gatsby’s house, making the night fine as before and surviving the laughter and the sound of his still glowing garden.

Does it mean only "disc"?

Than you