Wiktionary
n. (context databases English) A stage in the normalization of a relational database in which a database is in second normal form and all non-key attributes are mutually independent (no transient dependencies.)
Wikipedia
Third normal form is a normal form that is used in normalizing a database design to reduce the duplication of data and ensure referential integrity by ensuring that (1) the entity is in second normal form, and (2) all the attributes in a table are determined only by the candidate keys of that table and not by any non-prime attributes. 3NF was designed to improve database processing while minimizing storage costs. 3NF data modeling was ideal for online transaction processing (OLTP) applications with heavy order entry type of needs.