What does the KEY keyword mean?

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.

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.

column_definition:
      data_type [NOT NULL | NULL] [DEFAULT default_value]
      [AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY]
      ...

Ref: http://dev.mysql.com/doc/refman/5.1/en/create-table.html