Life-blood is a term commonly used to describe something that is essential for the existence or vitality of something else. There are several synonyms that can be used to describe this concept, including essence, backbone, mainstay, foundation, and vitality. Essence refers to the core or fundamental quality that gives something its identity. Backbone and mainstay highlight the idea of something being a crucial support or anchor. Foundation emphasizes the importance of something as a basis or starting point. Finally, vitality describes the energy and vigor that keeps something alive and flourishing. By using these synonyms, you can add nuance and variety to your writing while still conveying the essential meaning of life-blood.