Which type of organisms is characterized by having multiple cells?

Multicellular organisms are characterized by having multiple cells that are organized into complex structures and systems. These organisms can consist of tens, hundreds, or even trillions of cells, which work together to perform various functions necessary for life. This cellular organization allows for specialization, where different types of cells fulfill specific roles — such as nerve cells transmitting signals, muscle cells enabling movement, or blood cells transporting oxygen.

In contrast, unicellular organisms are composed of a single cell that carries out all life processes within that one unit. The distinction between multicellular and unicellular is fundamental in biology, as it highlights the complexity and diversity of life forms. Complex organisms generally refer to those that might be multicellular but can also imply a higher level of organization or an intricate body plan. Monocellular is synonymous with unicellular, reinforcing that it refers to organisms made up of just one cell. Thus, the defining trait of multicellular organisms is their multiple, functionally specialized cells that together contribute to the organism's overall survival and functionality.