An organism is any individual living system, such as an animal, plant, fungus, or bacterium. To be considered "alive," an organism generally must:

Physical features like a turtle's shell or a cactus's spines.

Actions like bird migration or nocturnal hunting.

The basic unit. Unicellular organisms (like yeast) are just one cell. Multicellular organisms (like humans) have trillions.

Organisms with complex cells containing a nucleus. This includes: Protists: Mostly single-celled (amoebas). Fungi: Decomposers (mushrooms, molds).