A group of cells performing a specialized function is called a tissue.

Here's a breakdown:

* Cells: The basic unit of life.

* Tissue: A collection of similar cells that work together to perform a specific function.

* Organ: A structure made up of different tissues that work together to perform a complex function.

* Organ System: A group of organs that work together to carry out major bodily functions.

* Organism: A complete living being made up of multiple organ systems.