Final answer:
An organ is defined as an anatomically distinct structure in the body made up of two or more tissue types that work together to perform specific physiological functions.
Step-by-step explanation:
A structure consisting of tissues organized to carry out a specific function defines an organ. The smallest independently functioning biological unit is a cell, which can organize into tissues. A group of tissues that collaborate to perform a certain task forms an organ.
Examples of human organs include the heart, lungs, and liver. In plants, examples include roots, stems, and leaves. It is these organs that work together to form organ systems within an organism, each system serving a major function or physiological need of the body.