Subject: What is a 'plasmon'?

I've seen some references to plasmons in stories on spacedaily.com.  But,
searching the web finds no layman's explaination about them--only technical
papers for condenced matter physisists, or a company that happens to be spelled
the same way.  Searching madsci shows that the word has never been uttered here
before now.

So, can you explain what is a plasmon to someone who knows the kinds of things
he sees on the Discovery Channel or reads in SciAm?

Re: What is a 'plasmon'?