Here’s a Lewis quote (not verified, someone had typed it into a blog) that shows something about what he (at least) meant by ‘allegory.’

“Human intellect,” says Lewis, “is incurably abstract…Yet the only realities we experience are concrete–this pain, this pleasure, this dog, this man. …. In the enjoyment of a great myth we come nearest to experiencing as a concrete what can otherwise be understood only as an abstraction. …. You are not looking for an abstract ‘meaning’ at all. If that was what you were doing the myth would be for you not true myth but a mere allegory. You were not knowing, but tasting; but what your tasting turns out to be a universal principle. The moment we *state* this principle, we are admittedly back in the world of abstraction. It is only while receiving the myth as a story that you experience the principle concretely. “When we translate we get abstraction – or rather, dozens of abstractions. What flows into you from the myth is not truth but reality (truth is always *about* something, but reality is that *about which* truth is), and , therefore, every myth becomes the father of innumerable truths on the abstract level.

(God in the Dock : Essays on Theology and Ethics — C. S. Lewis, Walter Hooper (Editor); Paperback.)