Although most people have an intuitive understanding of whether something is alive, we still lack of a concrete definition of life. What is life? This simple question is surprisingly difficult to answer; yet, it is fundamental to understanding life as a universal phenomenon and to search for it elsewhere. Defining life, despite significant effort invested on it, has been elusive. This may be due to limitations in our understanding of the fundamental components of life, or in the way we frame it to be empirically and iteratively tested. However, the very exercise of studying life’s most essential characteristics has revealed many exciting insights and may uncover its true nature in the future. In this chapter, we will discuss the historical definitions of life, their key features and how they help us search for life outside of Earth.