What is the process of obtaining pure water from sea water called?

Desalination (or distillation) is the process used to convert salt water into pure water.

Desalination/Distillation is one of mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today. In ancient times, many civilizations used this process on their ships to convert sea water into drinking water. Today, desalination plants are used to convert sea water to drinking water on ships and in many arid regions of the world, and to treat water in other areas that is fouled by natural and unnatural contaminants. Distillation is perhaps the one water treatment technology that most completely reduces the widest range of drinking water contaminants.

https://en.wikipedia.org/wiki/Desalination

Distillation - Separating a dissolved solid and a liquid. It is different from evaporation because both solid (residue/salt) and the liquid (distillate/water) can be isolated.