The American Heritage Medical Dictionary defines "living organisms" as beings who share the following characteristics:
An individual form of life, such as a plant, animal, bacterium, protists, or fungus; a body made up of organs, organelles, or other parts that work together to carry on the various processes of life.That last phrase saying "carry on the various processes of life" is the key to the meaning of "living". The processes of life include: birth, growth, evolution, death, transformation, metabolism, and all the other bodily functions that are inherent to maintaining a life. Hence, a living thing differs from a non-living thing in that, whether they are made of similar elements or not, the living things carry on life processes while the non-living thing does not. This being said, think about a human being (a living thing), and water. Water has molecules, so do we. Water moves, so do we. However, there is no life processes involved in the water cycle: Water simply flows. Therefore, living organisms are all living beings whose life depends of specific life-maintaining processes. Non-living things are basic elements that could help build and maintain life.