BiologyWise
Biology Wise: The 7 Unmistakable Characteristics of Life
Describes the seven characteristics that define life - cells, homeostasis, heredity, use of energy, reproduction, response to environmental stimuli, and evolution and adaptation.
Khan Academy
Khan Academy: Are Viruses Dead or Alive?
Are viruses alive or dead? How do we determine whether something is alive? Let's compare viruses to the 7 criteria researchers have set to determine if something is alive.