Personally I use a mixture of Hyman Minsky and Graham & Dodd.

An investment should earn its return from it's own income/cashflow stream. Roughly equivalent to Minsky's defintion of hedge finance. The main characteristic is that the return comes from the asset itself, so unlike Graham future growth can be reasonably considered (though growth is very hard to successfully project).

A speculative investment will provide a return only assuming a change in external factors not instrinsic to the asset itself. Mainly, an increase in the pricing multiple / decrease in discount rate / increase in the cost of carry. (commodities, venture capital, a lot of buyout transactions, etc)

A ponzi asset produces no significant cashflow or benefit and can only generate a return if a thrid party purchases it a higher price. Think of art, wines, tullips, etc, but even conventional assets classes can become ponzi assets if the pricing gets completely out whack with the underlying (the dot-com bubble comes to mind)