The Free On-line Dictionary of Computing (30 December 2018):

database normalisation
1NF
2NF
3NF
4NF
5NF
Codd's First Normal Form
fifth normal form
first normal form
fourth normal form
second normal form
third normal form

    A series of steps followed to obtain a database
   design that allows for efficient access and storage of data
   in a relational database.  These steps reduce data
   redundancy and the chances of data becoming inconsistent.

   A table in a relational database is said to be in normal
   form if it satisfies certain constraints.  Codd's original
   work defined three such forms but there are now five generally
   accepted steps of normalisation.  The output of the first step
   is called First Normal Form (1NF), the output of the second
   step is Second Normal Form (2NF), etc.

   First Normal Form eliminates repeating groups by putting
   each value of a multi-valued attribute into a new row.

   Second Normal Form eliminates functional dependencies on a
   partial key by putting the fields in a separate table from
   those that are dependent on the whole key.

   Third Normal Form eliminates functional dependencies on
   non-key fields by putting them in a separate table.  At this
   stage, all non-key fields are dependent on the key, the whole
   key and nothing but the key.

   Fourth Normal Form separates independent multi-valued facts
   stored in one table into separate tables.

   Fifth Normal Form breaks out data redundancy that is not
   covered by any of the previous normal forms.

   (http://bkent.net/Doc/simple5.htm).

   [What about non-relational databases?]

   (2005-07-28)