What is the term for a structure composed of two or more tissue types?

The term for a structure composed of two or more tissue types is "organ." In biological terms, organs are complex structures that perform specific functions and are made up of different types of tissues that collaborate to carry out those functions. For example, the heart is an organ that consists of muscle tissue, connective tissue, and epithelial tissue, all working together to pump blood throughout the body.

In contrast, "tissue" refers to a group of similar cells that perform a specific function, such as muscle tissue or nervous tissue. A "cell" is the smallest structural and functional unit of an organism, capable of performing life processes. "System" refers to a group of organs that work together to perform complex functions, such as the circulatory system or the digestive system. Therefore, the term organ specifically highlights the combination of different tissue types, making it the correct answer in this context.