Sustenance is food and drink that keeps a living thing alive and healthy. After a long hike, you need sustenance: water to rehydrate and a sandwich to restore your energy. Plants draw their sustenance from sunlight, water, and nutrients in the soil. A bear preparing for winter hibernation eats enormous amounts of sustenance to build up fat reserves.

Sustenance doesn't just fill your stomach, it sustains you. It provides what you need to keep going. Without adequate sustenance, your body grows weak and can't function properly.

People also use sustenance to describe non-physical things that keep you going emotionally or spiritually. A musician might say that making music provides sustenance for her soul. A reader might find sustenance in good books. In this sense, sustenance means whatever nourishes and strengthens you, whatever helps you thrive rather than merely survive. When you feel depleted after a hard day, you might need physical sustenance like a good meal, or emotional sustenance like time with people who care about you.