Yeast is classified as a fungus. The defining characteristics of fungi include being eukaryotic organisms that have a cell structure containing a nucleus, along with cell walls made primarily of chitin. Yeasts typically exist as single cells and reproduce asexually through a process known as budding.
In contrast, bacteria are prokaryotic organisms that lack a nucleus and have cell walls composed mainly of peptidoglycan. Protozoa are also eukaryotic but differ significantly from fungi in their modes of nutrition, movement, and cellular organization. Viruses are acellular entities that do not have cellular structures and require a host to replicate, which distinctly separates them from yeast and other forms of life.
Thus, identifying yeast as a fungus reflects its biological characteristics and functional roles in various ecosystems, including fermentation processes in food and beverage production.