Source

File: wp-includes/class-wp-term.php

public static function get_instance( $term_id, $taxonomy = null ) {
		global $wpdb;

		$term_id = (int) $term_id;
		if ( ! $term_id ) {
			return false;
		}

		$_term = wp_cache_get( $term_id, 'terms' );

		// If there isn't a cached version, hit the database.
		if ( ! $_term || ( $taxonomy && $taxonomy !== $_term->taxonomy ) ) {
			// Any term found in the cache is not a match, so don't use it.
			$_term = false;

			// Grab all matching terms, in case any are shared between taxonomies.
			$terms = $wpdb->get_results( $wpdb->prepare( "SELECT t.*, tt.* FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id WHERE t.term_id = %d", $term_id ) );
			if ( ! $terms ) {
				return false;
			}

			// If a taxonomy was specified, find a match.
			if ( $taxonomy ) {
				foreach ( $terms as $match ) {
					if ( $taxonomy === $match->taxonomy ) {
						$_term = $match;
						break;
					}
				}

				// If only one match was found, it's the one we want.
			} elseif ( 1 === count( $terms ) ) {
				$_term = reset( $terms );

				// Otherwise, the term must be shared between taxonomies.
			} else {
				// If the term is shared only with invalid taxonomies, return the one valid term.
				foreach ( $terms as $t ) {
					if ( ! taxonomy_exists( $t->taxonomy ) ) {
						continue;
					}

					// Only hit if we've already identified a term in a valid taxonomy.
					if ( $_term ) {
						return new WP_Error( 'ambiguous_term_id', __( 'Term ID is shared between multiple taxonomies' ), $term_id );
					}

					$_term = $t;
				}
			}

			if ( ! $_term ) {
				return false;
			}

			// Don't return terms from invalid taxonomies.
			if ( ! taxonomy_exists( $_term->taxonomy ) ) {
				return new WP_Error( 'invalid_taxonomy', __( 'Invalid taxonomy.' ) );
			}

			$_term = sanitize_term( $_term, $_term->taxonomy, 'raw' );

			// Don't cache terms that are shared between taxonomies.
			if ( 1 === count( $terms ) ) {
				wp_cache_add( $term_id, $_term, 'terms' );
			}
		}

		$term_obj = new WP_Term( $_term );
		$term_obj->filter( $term_obj->filter );

		return $term_obj;
	}