1. (adjective) urgently needed; absolutely necessary

• critical

2. (adjective) performing an essential function in the living body

• life-sustaining

3. (adjective) full of spirit

• full of life
• lively

1. (adjective) urgently needed; absolutely necessary

• indispensable

2. (adjective) performing an essential function in the living body

• essential

3. (adjective) full of spirit

• alive
• animated

4. (adjective) manifesting or characteristic of life

• alive
• live

1. (adjective) urgently needed; absolutely necessary

• criticality
• criticality
• criticalness
• criticalness
• cruciality
• cruciality
• indispensability
• indispensableness
• vitalness

2. (adjective) performing an essential function in the living body

• elan vital
• life force
• vital force
• vitality
• indispensability
• indispensableness
• vitalness
• vitalness

3. (adjective) full of spirit

• animation
• vitality
• energy
• vim
• vitality
• life
• liveliness
• spirit
• sprightliness
• verve
• vitality
• vitalise
• vitalize

4. (adjective) manifesting or characteristic of life

• animation
• vitality
• vitalise
• vitalize
• vitalness