Long-lasting volume can be defined as hair that is lifted and held away from the scalp and defies gravity and humidity over time.

Specifically, consumers who are styling their hair for volume tell us they are looking for:*

When consumers have volume style failure, they typically see:

* Internet Survey, April, 2002.

Why is it so hard for some people to keep great volume? Why are some of us struggling with lank, limp head-hugging hair while others seem to have full, bouncy hair all the time? The answer is physiological in nature - we inherited a genetic tendency toward fine or limp hair.

Several physiological factors affect volume. See Factors Influencing Hair Volume for more information.

The primary challenge in creating volume is creating a space, or air pocket, between the hair and the scalp. The most common areas where volume is desired are:

Root lift - increases apparent height and length of head)
Bang lift - keeps hair up and off face)
Side volume - hair doesn't cling to scalp)
End volume - hair looks free flowing or shaped without stringiness)

There are several techniques to "pump it up". See How To Get Long-Lasting Volume for more information.

From article:
What is long-lasting hair volume?