Brad, I've been thinking about the same subject quite a bit recently and agree with your post and many of the comments here. On anything like this its always helpful to me to go back to early thinkers. Graham, as ever, is insightful here.

I'll paraphrase - An investment is something that, based on thorough analysis promises safety of principal and an adequate return, anything else is speculative.

While dated by the notion of safety of principal, if you insert "acceptable risk", you have a great working definition.