Wild yeast is used by brewers to refer to the yeast present at low levels in the environment, as opposed to the yeast pitched intentionally by the brewer. Ordinarily, this is considered undesirable, but some traditional beers, especially Belgian ales, rely on wild yeast for their unique character.
Specific strains of yeast other than the traditional strains used in most ale and lager brewing, such as brettanomyces and non-standard species of saccharomyces, are sometimes referred to as wild yeasts even if they are cultures and pitched intentionally. The terms is also sometimes used, somewhat inaccurately, to refer to non-yeast organisms traditionally found in spontaneously fermented beerm such as the bacteria pediococcus and lactobacillus.
 Spontaneous Fermentation
The first brewers did not have access to Wyeast smack packs. In fact, for many years, brewers did not even know about the existence of yeast; for example, in the famous Bavarian beer purity law, the Reinheitsgebot, yeast is not included as a permitted beer ingredient. Instead, early beers got their yeast in one of two ways: from the addition of part of a previous batch of beer, such as through krausening, or from the environment, through spontaneous fermentation.
Spontaneous fermentation makes use of the wild yeasts in the environment. If a sterile wort is left in a sanitized container exposed to the air, eventually wild yeast and bacteria found naturally in the air will start reproducing in the wort and fermenting it into beer.
This is a hit-and-miss process at best. However, once a good mix of yeast and bacteria occurs by chance, it will remain in wooden casks or fermentation vessels and in the local environment, allowing more consistency in the finished beer. Many Belgian breweries still operate with this kind of spontaneous fermentation.
 Commercial Wild Yeast Cultures
For homebrewers who are less adventurous or who want to reproduce specific strains, wild yeast and bacterial cultures are available of some of the major strains of wild yeast and bacteria found in Belgian beers.