Freeze drying (lyophilization) relies on sublimation, which is the direct transition of ice (solid) into water vapor (gas) without passing through the liquid phase.
For water to sublime, both temperature and pressure must be kept below the triple point of water ($0.01^\circ\text{C}$ and $611.65\text{ Pa}$ or $0.006\text{ atm}$). Above the triple point, ice melts into liquid water instead of sublimating directly into vapor.
