What's the difference between using INDEX vs KEY in MySQL?

I know how to use INDEX as in the following code. And I know how to use foreign key and primary key.

CREATE TABLE tasks ( 
  task_id INT UNSIGNED NOT NULL AUTO_INCREMENT, 
  parent_id INT UNSIGNED NOT NULL DEFAULT 0, 
  task VARCHAR(100) NOT NULL, 
  date_added TIMESTAMP NOT NULL, 
  date_completed TIMESTAMP, 
  PRIMARY KEY (task_id), 
  INDEX parent (parent_id), 
  ....

However I found a code using KEY instead of INDEX as following.

...
KEY order_date (order_date) 
...

I could not find any explanation on the official MySQL page. Could anyone tell me what is the differences between KEY and INDEX?

The only difference I see is that when I use KEY ..., I need to repeat the word, e.g.
KEY order_date (order_date).

Solution 1:

There's no difference. They are synonyms.

From the CREATE TABLE manual entry:

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

Solution 2:

Here is a nice description about the "difference":

"MySQL requires every Key also be indexed, that's an implementation detail specific to MySQL to improve performance."

Solution 3:

Keys are special fields that play very specific roles within a table, and the type of key determines its purpose within the table.

An index is a structure that RDBMS(database management system) provides to improve data processing. An index has nothing to do with a logical database structure.

SO...

Keys are logical structures you use to identify records within a table and indexes are physical structures you use to optimize data processing.

Source: Database Design for Mere Mortals

Author: Michael Hernandez