I believe one of the best definitions of clutter comes from Karen Kingston's book Clear Your Clutter With Feng Shui. Here are her four categories of clutter:

• Things you do not use or love
• Things that are untidy or disorganized
• Too many things in too small a place
• Anything unfinished

These categories help us define for ourselves what is cluttering our own home.
I prefer this over other people tell ing me something is clutter and I should get rid of it, when for me it isn't clutter. For example, most people would say that old magazines are clutter. Makes sense, except I have collected many issues of a particular decorating magazine that I love and regularly peruse again and again because I find doing so is peaceful to me. Old issues of this magazine also often sell for three times or more the regular cover price on eBay. I use and love this magazine, therefore it's not clutter. However I recently tossed out many other old magazines I've collected because I wasn't using them and didn't love them.
When defining whether your own stuff is clutter simply ask yourself if it fits into one of these four categories.