The word Epicurus uses is εναργής.
It has two primary definitions:
- visible, palpable, in bodily shape, properly of gods appearing in their own forms (in Homer); so of a dream or vision; ex., ἐναργὴς ταῦρος "in visible form a bull, a very bull"
- manifest to the mind's eye, distinct
He can't mean the first since he's fairly adamant that the gods don't interact with humans. But the second definition coincides with his contention (and the idea of the prolepseis) that the gods are apprehended by the mind only. That also sets up a nice contrast with the first definition's use by Homer in describing the Olympian gods appearing "in visible form." Homer's gods were εναργής in one sense of the word; Epicurus's in the other sense.