Consider these examples:
Warren Buffett: a good business that can be purchased for less than the discounted value of its future earnings.
George Soros: an investment that can be purchased (or sold) prior to a reflexive shift in market psychology/fundamentals that will change its perceived value substantially.
Benjamin Graham: a company that can be purchased for substantially less than its intrinsic value.
A few more examples:
The Corporate Raider: companies whose parts are worth more than the whole.
The Technical Analyst: an investment where technical indicators have identified a change in the price trend.
The Real Estate Fixer-Upper: run-down properties that can be sold for much more than the investment required to purchase and renovate them.
The Arbitrageur: an asset that can be bough low in one market and sold simultaneously in another at a higher price.
The Crisis Investor: assets that can be bought at fire-sale prices after some panic has hammered a market down.
Coming to your definition of a good investment is easy - if you're clear about the kinds of investments that interest you and have clarified your beliefs about prices and values.