Originally Posted by Drunken Chicken
Nope, it's when instead of sitting down and pedalling to keep the bike on it's rear wheel, you simply stand up, pull the bars up, stay stood up and balance the bike on the rear wheel (while moving) without spinning the crankset (that would turn it into a coaster wheelie).
Clarifications (the front wheel never touches the ground in any of these manuevers):
sitting down, pedaling = wheelie.
standing up, pedaling = also a wheelie, aka power wheelie.
standing up, coasting = manual.
sitting down coasting = also a manual, aka coaster wheelie.
These are variable definitions depending on your location and your crew, but I think the general consensus is pedaling = wheelie, no pedaling = manual.