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?

mysql indexing
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