A hierarchical organization. The sum total of all living things. A set of energy driven, self replicating chemical reactions. The best adapted individuals tend to produce the most offspring.
Any population has the potential to produce more offspring than the environment can support.
Rocks have mass. Enzymes can catalyze chemical reactions. Tissues are organized into organs. A given element has 8 protons. Some of the atoms of this element have 8 neutrons and some have 10 neutrons.