Source

File: wp-includes/category-template.php

function term_description( $term = 0, $deprecated = null ) {
	if ( ! $term && ( is_tax() || is_tag() || is_category() ) ) {
		$term = get_queried_object();
		if ( $term ) {
			$term = $term->term_id;
		}
	}

	$description = get_term_field( 'description', $term );

	return is_wp_error( $description ) ? '' : $description;
}