Data redundancy is when information from a field is recorded in multiple tables within a database or spreadsheet unnecessarily. Attaching a buyer's name to every individual item purchased, instead of the transaction as a whole, is a form of redundant data.
Data redundancy often clutters the database with unneeded information, making the retrieval of the desired info less efficient and swelling the amount of resources needed to search the database. Redundant data can cause inconsistencies, as a particular entry may pick up multiple attributes, causing it to be unusable when looking for any one singular attribute. Over time, redundancies create a database corruption, by making the information within it unusable. Using database normalization can prevent redundancy by creating one singular, efficient table for editing fields, and distributing the information to the rest of the database from it.