Welcome, stranger from spaceland, to the fourth dimension! "The fourth dimension?", you may ask. "Surely, you mean time?" But no, this is about spacial dimesions.
How could that be? Well, think about time: It is impossible to go back in time. It moves in only one direction. So it is sort of only half a dimension.
Futhermore, to anticipate things a little, in this tutorial you will get to know a two-dimensional being, living in a plane. Would't time pass for this being, too? After all, life is not conceivable without time.
Thus, we may define time not as the fourth, but the upmost dimension, no matter in how-dimensional space we are.
But how can we define a dimension in general? Well, that's easy: In this course at least, we will use the term "dimension" in the sense of "freedom of movement"

Read on to learn more...

Imagine being stuck in a traffic jam. You can neither move forward nor backward, you cannot overtake, that is move to the left or right, and also if you might wish you could, cou can't take off into the air and fly away. You are constrained to one single place: You are in a zero-dimensional situation.

What about a train? A train can only move forward and backward on rails.
It is in a one-dimensional situation.

Now take a ship. Obviously, it can move in two dimensions on the water, which can be considered as a plane. Thus it is in a two-dimensional situation.

A plane, on the other hand, can move in whatever direction it wants.
It can move in three-dimensional space.