A unicellular organism is made up of a single cell, which performs all the functions necessary for life. These organisms can be found in various environments, including soil, water, and even inside other living beings. Examples of unicellular organisms include bacteria, amoebas, and yeasts. Unicellular organisms reproduce primarily through a process called binary fission, where one cell divides into two identical cells. They can be classified into different categories, such as prokaryotes, which lack a nucleus, and eukaryotes, which have a defined nucleus. Despite their simplicity, unicellular organisms play crucial roles in ecosystems and human health.