Scientific definition of pumps
A pump is a buildup of blood plasma in an individual muscle or groups of muscle. The best way to describe it is this....
Picture your muscle as a ballon, but in the ballon you have many layers of tissue, which are connected to a capilary bed. So that the cap. bed is fed by an artery, that feeds the muscle. During a workout plasma is forced into the muscle via increased blood pressure. The heart rate increases, stroke volume and VO2 go up. Now, this ballon is connected to the capilary bed, which has two connections to it, an artery and a vein. As more plasma fills the muscle, it takes more blood pressure to fill the muscle and push plasma out and into the capilary, because the capilary is only 1 cell thick and things pass 1 cell at a time or atleast in very small increments. So, basically what you have is called a blood plasma shift. You have plasma entering and filling the muscle giving it a temporary hypertrophy, increased size, possible pain. The resulting pain and/or size is deemed a "pump." So, if you are working a muscle with full force 90% and above for the rep range, you have a feeling of growth and pain, the "pump" we all seek as a biofeedback.
Hope that helps,
Rex
BTW - i suck at spelling. :)