Source

File: wp-includes/taxonomy.php

function add_term_meta( $term_id, $meta_key, $meta_value, $unique = false ) {
	if ( wp_term_is_shared( $term_id ) ) {
		return new WP_Error( 'ambiguous_term_id', __( 'Term meta cannot be added to terms that are shared between taxonomies.' ), $term_id );
	}

	return add_metadata( 'term', $term_id, $meta_key, $meta_value, $unique );
}