'tax_query' => array(
    array(
        'taxonomy' => 'city',
        'field' => 'slug',
        'terms' => array( nyc, boston, london ),
         )
)

// get all terms in the taxonomy
$terms = get_terms( 'city' );
// convert array of term objects to array of term IDs
$term_ids = wp_list_pluck( $terms, 'term_id' );

// proceed with tax query
$args = array ('tax_query' => array(
    array(
        'taxonomy' => 'city',
        'field' => 'term_id',
        'terms' => $term_ids,
         )
)
);

$city_posts = new WP_Query( $args );

$args = array ('tax_query' =>
     array(
         array(
              'taxonomy' => 'city',
              //'field' => 'term_id', // <-- not needed, default
              //'terms' => $term_ids, // <-- not needed
              'operator' => 'XXX'     // Use a non-existing operator
     )