Random co-worker chimes in on a mailing list with a very accurate summation of Adobe (former Macromedia) Flash:

“Browsing without Flash is like going to the movies without ringing cell phones and crying babies. Every once in a while you miss out on a really cool ring tone, but it’s nothing to lose sleep over.”

When it comes to defining Flash, I think this list of synonyms on Dictionary.com does a much better job than Wikipedia every could.