mysql - What does the KEY keyword mean?

Question

In this MySQL table definition:

CREATE TABLE groups (
  ug_main_grp_id smallint NOT NULL default '0',
  ug_uid smallint  default NULL,
  ug_grp_id smallint  default NULL,
  KEY (ug_main_grp_id)
);

What does the KEY keyword mean? It's not a primary key, it's not a foreign key, so is it just an index? If so, what is so special about this type of index created with KEY?

Answer 1

Quoting from create-table - indexes andkeys

{INDEX|KEY}

So KEY is usually an INDEX

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.