A beer can only be labeled Lambic, when it is spontaneously fermented, which means wild yeast is used.  This wild yeast is harvested by exposing the wort (the cooked grain liquid) to the open air.

The spontaneous fermentation is the first known method to ferment sugars into alcohol, and has been used for over 30,000 years. Technology made it possible to harvest the yeast, and to use it in different, new batches.

However, several Belgian breweries replenish their stock of wild yeast on a regular basis.