The use of is when and is where in explaining and defining something is a common error in speech and writing. Rather than misuse an adverbial clause (Anemia is when the blood is deficient) employ a noun or a noun with modifiers (Anemia is a disease in which deficiencies appear in the blood.) To say, for instance,"Stealing is where you take..." instead of "Stealing is the act of taking..." is to be awkward, if not illiterate.
Dictionary of problem words and expressions. Harry Shaw. 1975.