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).