Freeze drying of a substance is known as:

Freeze drying of a substance is known as lyophilisation. This process involves removing moisture from a material after it has been frozen, allowing the substance to retain its structure and properties. During lyophilisation, the frozen water in the material transitions directly from a solid to a vapor phase, a process known as sublimation. This method is widely used in the preservation of biological samples, pharmaceuticals, and food products, as it effectively reduces water content while preserving the characteristics of the original substance.

The other terms mentioned have different meanings; dehydration refers to the removal of water from a substance but does not necessarily involve freezing, evaporation is the process of converting liquid into vapor without a phase change like freezing, and condensation refers to the process of vapor converting back into liquid. Therefore, lyophilisation specifically captures the essence of freeze drying more accurately than the other options.