"Brett" for short, refers to a spoilage organism of the yeast genus that gives wine a barnyardy aroma and distinctive "stink." For while low levels of the contaminant can render a wine more interesting and complex, higher levels most certainly spoil it, overriding its character and terroir. A hotly debated question for winemakers and wine drinkers alike is concerning how much Brettanomyces (or Dekkera, a similar organism) can be present before spoilage occurs or whether it is altogether undesirable. After all, certain people abhor "Bretty" wines for their aromas that have been described as wet wool, band-aid and droppings of all kinds.
A fungal infection. The Brettanomyces fungus can originate in the vineyard but some wineries are chronically contaminated, the organism living in oak barrels or even on the wooden structure of the winery itself. As a consequence the wines from this vineyard can bear a Brett profile - farmyardy, horsy, sometimes metallic aromas - year after year. Examples of châteaux which are renowned for Brett contamination are Beaucastel (Châteauneuf du Pape, Rhône Valley) and Talbot (St Julien, Bordeaux).
A wild yeast strain that occurs naturally in wineries and vineyards. Low levels of infection can add complexity to a wine. High levels are perceived by most as a fault.