Any of a genus of usually unicellular yeasts; such as, a brewer's yeast, that are distinguished by their sparse or absent mycelium (loose network of the delicate filaments that form the body of a fungus) and by their facility in reproducing asexually by budding.

1. Any of several single-celled yeasts belonging to the genus Saccharomyces that lack a true mycelium and many of which ferment sugar.
2. A genus of yeast fungi, including brewer's and baker's yeast, as well as some pathogenic fungi, that cause such diseases as bronchitis, moniliasis, and pharyngitis.