"Consistency - This one doesn't really belong on ACID as a property of databases, as it is a property of the application." Damn, that is not good on so many levels.

First of all, ACID are not properties of a database, they are properties of database transaction. Article kind of says it, but I feel it is phased ambiguous. Consistency is the property of a database transaction which can be easily illustrated by foreign keys.

    AUTHORS
    | author_id | name |
    |         1 | Mary |
    |         2 | John |

    ARTICLES
    | article_id | author_id | title |
    |         11 |         2 | About small animals |
    |         12 |         1 | About big machines |

Now ARTICLES cannot have author_id non existent in AUTHORS. If a tuple is inserted in ARTICLES, then author_id should reference a valid tuple from AUTHORS. If a tuple is deleted from AUTHORS, then all ARTICLES tuples referencing said AUTHORS tuple should be deleted too; or deleting from AUTHORS should be rejected. That is consistency and that is a property of a database transaction. Transaction must fail if one tries to insert an article with author_id equal 5. At the end of transaction article 11 must be deleted if author 2 was deleted.

Now real life databases are much more complex, but idea is basically the same.

Also, consistency is between tuples. Not having "31-February-2001" as a date is integrity, not consistency.