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*. ```mysql 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 NULL, PRIMARY KEY ( task_id ), INDEX parent ( parent_id ) ) ``` However I found a code using `KEY` instead of `INDEX` as following. ```mysql CREATE TABLE orders ( order_id int unsigned NOT NULL AUTO_INCREMENT, -- etc 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 )`.