What term describes an association of many cells that together form a specific function?

The term that describes an association of many cells working together to perform a specific function is "tissue." Tissues are composed of cells that share a common structure and function, and they are organized in a way that allows them to carry out particular activities within an organism. For example, muscle tissue is made up of muscle cells that enable movement, while epithelial tissue forms protective layers on surfaces of organs.

In contrast, an organ is a structure made up of different types of tissues working together to carry out specific functions, such as the heart or liver. A cell is the basic unit of life and does not refer to a collection of cells. An organ system is a group of organs that work together to perform complex functions, such as the digestive system. Thus, "tissue" is the most appropriate term for describing a collection of similar cells that cooperate to fulfill a common purpose.