Most people define clutter as too much stuff, which is certainly accurate, as long as you can define what's "too much." Quantity is flexible; function is the more important issue. Is it physically in your way? Is it a constant annoyance in the back of your mind? I'd say that you have too much when your stuff gets in the way of living your life the way you want to live it.
Some people can amass a ton of stuff and be quite content: collectors. They enjoy what they own, they care for it, display it (as much of it as they can), and are eager to share it and their experiences collecting it with others. Here are two completely different examples of collectors: one is quite traditional, while the other collected cast-offs to create art.
Archaeologist Andrea Ludden’s collection of over 40,000 pairs of salt and pepper shakers is housed in two museums, one of them created by her and her family. Click here to read more and be sure to view the excellent slide show of examples from the collection.
High school teacher, writer and artist Victor Moore constructed his Junk Castle from salvaged items such as sheet metal, tin, washing machine parts, dryer doors, miscellaneous housings, bedsteads, and the door from a '52 Oldsmobile. Another interesting slide show is at this link.
Is your stuff as well cared for, as beautiful, or as useful as the items in these collections -- or is it just clutter?