First of all I'll give the juggling purist's definition of juggling. They think you are only really juggling if:

n > b

That is it. I bet that gave you a surprise. n = objects and b = hands. So if you are juggling with one hand the amount of objects you have to be throwing must be two or more. Using two hands? Anything under three objects just won't cut it.

But what about contact juggling? I hear you say. What about diabolo? What about poi, yoyo, balancing, plate spinning, etc, etc, etc? I agree, that definition is a very exclusive. For those to be included in a definition of juggling I'll give you the opposite view, the inclusive's definition:

Juggling = any kind of object manipulation.

Now I hear you say things like: So manipulating the mouse on my computer is juggling? I think we could all agree it isn't but where do we draw the line?

Other people have other definitions from the very unhelpful "Juggling is what jugglers do and nobody else does" to the amusing "Doing that which doesn't need to be done in the most difficult way possible." The first one I think we can disregard until we have answered our first "what is juggling" question and I think it is best to forget the second because that would include "running the London Marathon dragging a millstone by a rope around your neck" and "reading this column with your eyes closed" as juggling.

I think a good way of scientifically answering "What is juggling?" is to make a short list of object manipulation that we all agree is definitely not juggling, then an opposite list.

Not juggling:
mouse control
millstone marathon running
playing a musical instrument
flying a kite
doing up a button
riding a bike
sex

Juggling:
throwing lots of balls up and catching them
bouncing a ball up and down on your head (works with the n > b rule)
trying to manage lots of stressful things at once (juggling work, wife, kids and your bit on the side)

And now some things in the grey area:
yoyo
poi
devilstick
contact juggling
balancing
plate spinning
going to a juggling convention, drinking lots and falling over on stage (jugglers do it and non-jugglers do not)

Hmmm, I'll get back to you on these. In the meantime, you work it out, it can't be that important.