Deepti actually does not mean lamp or candle in any north Indian language. It does mean light as in the thing that comes from a lamp, not light meaning a lamp. Deepavali and Deepti do share the same root, but they come from different grammatical forms: deepa (with the final a unstressed and unpronounced in many languages) means a source of light, but deepti only means the light itself. There are also meanings like lac and brass for deepti in Sanskrit, but they are rarely used. Deepti also applies to brightness as in beauty. The male form Deepta, also used as a name, does mean burning or bright, and hence can be used for a lamp. Finally, the aavali bit of deepaavali means a row (from a bunch of related roots vr, bal, val, all connected with to- and fro- motions), and thence the name of the festival.