You knew SOMEONE would ask/mention it eventually, so I'll throw it on out... the 4th dimension. What are your thoughts on it? (Don't hurt me!)

There are two (2) fourth dimensions, one of time, and the other of space. To distinguish between the two of them, I call the time dimension 'the 4th dimension', and the 4th 'spatial' dimension, I call the "fifth" dimension. The time dimension as discovered by Einstein (or was it Lorentz?)is the dimension which permeates throughout the spatial dimensions. Time is not absolute, it is relative to the observer who will each measure generally different times for the same events, as a function of their relative speeds, and both will be correct in their measurements. The 4th spatial dimension (i.e., the 5th) is even more difficult to comprehend. It is one of the 10 spatial dimensions of M-Theory, most of which are curled up so small (less than 10 to the minus 30 centimeters) that they cannot be observed or penetrated, except by high frequency gravity waves, and by the basic smallest constituent of matter, vibrating 'strings'. I say most are small, but the 5th may be large, such that once penetrated, it may be the gateway to parallel universes forever hidden from our view.

Oh, good, I was waiting for someone to mention time before I jumped in, myself! The interesting thing is, we actually live in four dimensions. The dimension of time is just as important as length, width, and depth. If you don't exist in time, you don't exist...

Actually, there may be many more than 4 dimensions. As Gerry hit on, dimensions may be too small or large for us to conceive of, but that doesn't mean that they don't exist.

For example, draw a line on a piece of paper, then stand back and look at it from a distance. From far away, it appears the "line" has only 1 dimension-length. However, as you get closer and closer to it, you notice that this so-called line also has a measurable height, so there are now 2 dimensions. Look even closer (maybe with a microscope), and you'll see that the line actually has depth too. These extra dimensions that the line exists in aren't readily seen from certain vantagepoints, but they do exist.

Don't quote me on this, but there may be actually be particles that exist in higher order dimensions that we can't directly detect in the 4 dimensions that we're familiar with.

Wow, that was a dumb slip-up on my part, shoulda said "more than"