An organ is defined as a structure made of different tissues that work together to perform a specific function. For example, the heart is an organ composed of muscle tissue (which helps pump blood), connective tissue (which provides structure), and nerve tissue (which controls heartbeat). This composition of various tissue types allows the organ to fulfill its role efficiently within the body.
In contrast, a group of similar cells defines a tissue, not an organ. A collection of proteins refers to macromolecules that may serve various functions in the body but do not constitute an organ. A system of interacting organs refers to a more complex structure, such as the digestive system or respiratory system, which encompasses multiple organs working together, but this is not the definition of a single organ itself.