Individualization is a process in homeopathy where an individual separated from other people (depending on his symptoms) suffering from same pathology.

Suppose three people in a family are suffering from malaria with common symptoms of malaria viz fever with chills followed by perspiration followed by coldness of body.
So these symptoms help to diagnose the disease. But in homeopathy we say that every individual is different from other and selection of the remedy should be based on his own unique symptoms.
Out of these three people suffering from malaria in the same family, one is thirsty, other is thistles. First one is irritable and third one experience a lot of fears. One wants to be alone and other needs company!
So people suffering same pathology have different symptoms and remedy is selected on the basis of these uncommon symptoms. 
This is called as “individualization”.