ItemId, the definition?

One day I will discover what an ItemId is.

It gets tacked on to the end of almost every Joomla! URL I've seen.

Like Home, the mother of all URLs:

http://127.0.0.1/joomla/index.php?option=com_frontpage&Itemid=1

Or News, another important destination:

http://127.0.0.1/joomla/index.php?option=com_content&task=section&id=1&Itemid=2

But as this thread at Joomla! Forums tells me, there may be no easy definition, but I'm not losing hope.

I am creating my own little Joomla! Glossary to celebrate when I discover simple definitions for complicated little things like ItemId!