What is De-normalization?

De-normalization is the process of attempting to optimize the  performance of a database by adding redundant data. It is sometimes  necessary because current DBMS’s implement the relational model poorly. A  true relational DBMS would allow for a fully normalized database at the  logical level, while providing physical storage of data that is tuned  for high performance. De-normalization is a technique to move from  higher to lower normal forms of database modeling in order to speed up database access.