What's an "instance"?

This is one of those geeky terms that’s slowly wiggling its way into
mainstream English.

Consider that in discussing the term “instance” I’ll be able to talk
about World of Warcraft as an example.

An instance is one of something you might have more than one of.

For example, in Windows click on Start,
Run, type in “notepad” and press
“OK”.

Now do it again.

You now have two instances or copies of Notepad running.
Each is the same program, but each can be used completely independently
to view or edit different files at the same time.

Ultimately, that’s all instance really means.

Some applications are what’s called “single instance”. That means
that if you try to run more than one copy of the program, the second
says “hey, there’s an instance of me already running!”, and brings that
previous instance forward while closing the attempt at a second.

Now, about World of Warcraft (and possibly other multiplayer games)
…

WoW has what are called “instance dungeons”. This means that when
you or your group enter a dungeon you get a copy – an instance – of
that dungeon all to yourselves. If another group enters the same
dungeon, they get another, different copy, where only their players,
and not yours, exist.

If that meaning is making it into mainstream gaming, I expect we’ll
be seeing it more frequently elsewhere as well.