A permanent link to an item (a blog post, an attachment, ...). The default structure is "?p=123", but WordPress can handle "pretty permalinks" and will rewrite them internally to the former version. If you have a question about special rewrite structures you should use the [url-rewriting] tag.
Is it possible to create a rewrite in WP dynamically such that no flushing is needed? i.e. not storing the rewrite in the database? Only defined in my plugin code?