What is the pump?

In one line

The pump is the short-lived swelling of a muscle during or right after a workout, as blood and fluid rush into it.

What is the pump?
As in 'I got a great pump today'
After several sets of arm exercises, your arms feel swollen and tight. At the gym people call this 'getting a pump'. This post explains what the pump during a workout is, why it happens, and how long it lasts.
In one line
A muscle briefly swells while you train
The pump is the brief swelling of a muscle during or right after a workout, as blood and fluid rush into it. It is a word bodybuilders use a lot.
First, blood rushes to the muscle
Working muscles get far more blood
The body sends more blood to the parts that are working. During exercise, the blood vessels in the muscles widen and blood flow to the muscles rises sharply. During all-out exercise, the blood going to the muscles is about 10 times the resting amount.
Water from the blood moves into the muscle
So the muscle swells
When a lot of blood rushes in, some of the water in it passes through the walls of the tiniest blood vessels (capillaries) into the muscle. With more water inside, the muscle swells up. That is the pump.
How much does it swell?
About 8% thicker after one session
Right after one session of knee extensions, the muscle at the front of the thigh was about 8% thicker than before. After 6 weeks of the same exercise, the muscle had actually grown by a little over a third of that pump.
How long does it last?
Over 1 hour, back to normal the next day
After heavy leg exercise, the thigh muscles were still thicker than before more than 1 hour later. Measured again the next day, they were back to their original thickness. The pump is a change that comes and goes on the same day.
Common misconception
Do you need a big pump to grow?
A review of muscle-growth research found no evidence that the pump makes muscles grow. People whose muscles got much more blood flow after exercise did not grow more muscle when they trained the same way. The pump shows that blood and water rushed into the muscle that day.
Summary
Blood and water briefly swell the muscle
The pump is the brief swelling of a muscle during or right after exercise, as blood and fluid rush in. It is gone by the next day, and there is no evidence that the pump makes muscles grow. Related terms: pump-up (the pump right before going on stage), muscle soreness.