Any yeast strain which was not deliberated introduced into the wort by the brewmaster. Though most wild yeasts are harmful to the beer, a very small minority of them are not.

Yeast that naturally occurs on grape skins.

Saccharomyces that haven't been hanging around man long enough, so they're not much good for baking bread or brewing beer, wine or mead.

Any yeast that is introduced accidentally into wort or beer from the environment.

Any yeast strain that is not deliberately selected and introduced into the beer by the brewer.

The only kind of yeast used when making authentic sourdough. Don't misinterpret that; there are many varieties of wild yeast, each with its own characteristics. Several different kinds of wild yeast can be found in a single starter. Unlike commercial yeast that you get at your local supermarket, wild yeast prefers an acidic environment. It has to, to survive the acids produced by the lactobacilli. (see lactobacilli: see wooden spoons)

Any airborne yeast. Now available in cultured form.