Clutter can be both. Stuff that's not decorative, if you're not using it and not going to use it, I'd find it a new home. I'm in the process of doing that.

IMO, how things are displayed can sometimes appear as clutter. You can have lots of items and if they're shown in a way that's pleasing to the eye you can get away with it.

I think clutter can be external (on tabletops, the floor, etc) and internal (closets and drawers).

To me, a home is "cluttered" when there is no place for the eyes to rest as it goes around the room. Rest is important!

Every wall does not need to be covered in artwork, each table does not need to be covered with accessories, sofas and beds don't need tons of pillows, and things don't need to be sitting on the floor next to every chair and in every corner.

Things that no longer have a useful function, do not please me when I look at them or make it hard for me to find something. That is my definition of clutter.
I decluttered 4-5 years ago and work hard to keep up the habit. I tend to be sentimental about objects and always think I will get that old table/lamp/tray/picture frame painted. I think twice now before I bring anything new (or old!) into the house. When my home is uncluttered I think it helps me focus on what I really need to accomplish.

More than what I have.
I have organized, stylish rooms.
Garage not included. Dh's domain.

Clutter is stuff that needs to be put away, as in "my desk is cluttered." Stuff that's in the right place isn't clutter, as in "my bookcase is chock full of books."

I like suero's definition. I think of clutter as stuff that isn't put away, or isn't wherever it is for a purpose. Looking good/decorative is a purpose.

I am pretty good with not having visible clutter--more than one person has come to my house and asked me "Where is all your STUFF?" -- but I do tend to let things pile up behind closed doors in cupboards and closets that are infrequently used.

My goal is to not add any more "clutter hiding" furniture and to reduce the pieces I already have. I recently bought a cabinet from Craigslist that I painted for use in my entryway. I removed the doors so now it's an open bookcase. That way, I know I will use the space as display-area- storage rather than scary-clutter-behind-a-door storage.

Clutter is my kitchen counter once the kids empty the mounds of papers out of their booksacks every afternoon.

I am a minimalist. It is a lifestyle that allows me to look at uncluttered flat surfaces in my home and enjoy them for what they are. I don't force it on anyone else, but it is the way we live. No extra furniture, no extra adornments, and just clean and clear. We use color on the walls and on furniture to keep it interesting, and we live in a 2,000 sq ft home that we love. My excessives are in the garden! Never too much color out there.