The spectacle (eye cap or brille) is the clear scale that covers and protects a snake's (and some geckos') lidless eye — it sheds along with the skin.
This is why a snake's eyes turn cloudy or blue before a shed. A retained eye cap — an eye cap that fails to come off with the shed — is a common problem from low humidity and can damage the eye; correct humidity and see a vet if it persists.