velocity

[jby]

In time dilation and length contraction, there is a velocity term.
How do we measure velocity at the first place?

[chroot]

Relative to some other object, which you assume to be stationary.

- Warren

[hemmul]

Actually, any reference frame is defined in a way, that in each point of space you have a spy - a being with a clock. Say an object is traveling in the space. Consider the following:
first event the object passes the coordinate (x1,y1,z1) of the space. the spy sitting at that point remembers the time of this event: t1
second event the object passes the coordinate (x2,y2,z2) of the space. the corresponding spy remembers the time of this event: t2
Now, when t2->t1, the instant velocity is defined as:
vx=(x2-x1)/(t2-t1);
vy=(y2-y1)/(t2-t1);
vz=(z2-z1)/(t2-t1);

Practically, there are no spies [:D] rather there are clock associated with a reference frame, relatively to which you want to determine the velocity [t)]

hope this answered your question :)