Question

define unicellular organisms​

Answer 1

unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Prokaryotes include bacteria and archaea. Many eukaryotes are multicellular, but the group includes the protozoa, unicellular algae, and unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8–4 billion years ago

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Answer 2

The organisms which are made of only one cell are called unicellular organisms.
They multiply by various methods like binary fission, etc.
Example - Amoeba, etc.