Source

File: wp-admin/includes/class-wp-comments-list-table.php

public function get_columns() {
		global $post_id;

		$columns = array();

		if ( $this->checkbox ) {
			$columns['cb'] = '<input type="checkbox" />';
		}

		$columns['author']  = __( 'Author' );
		$columns['comment'] = _x( 'Comment', 'column name' );

		if ( ! $post_id ) {
			/* translators: Column name or table row header. */
			$columns['response'] = __( 'In response to' );
		}

		$columns['date'] = _x( 'Submitted on', 'column name' );

		return $columns;
	}