array key: columns
The columns array item defines the columns that will be displayed when rendering the list table, Aside from the checkbox definition, column array keys must match the array keys of the items, or rows, returned by the list table query when the get_data array is formed with the database query.
$list_table_definitions['columns'] = array('cb' => '<input type="checkbox" />', 'make' => "Make", 'model' => "Model", 'mpg' => "MPG", 'cylinders' => "Cylinders", 'weight' => "Weight", 'model_year' => "Model Year", 'country' => "Country");