What is Denormalization?

If we talk About the Denormalization, it is a database optimization technique in which we add redundant data to one or more tables. It can help us avoid costly joins in a relational database. Attention that denormalization does not mean not doing normalization. This is an optimization technique that is applied after doing normalization.
Via a traditional normalized database, we store data in separate logical tables and attempt to minimize redundant data. We can try to keep only one copy of each piece of data in the database.
For instance, in a normalized database, we might have a Courses table and a Teachers table.Each entry in Courses would store the teacherID for a Course but not the teacherName. If we need to retrieve a list of all Courses with the Teacher name, we would do a join between these two tables.