Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. One million parsecs.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A unit of distance equal to a million parsecs
Etymologies
Examples

The expansion rate became known as the Hubble constant; Hubble thought it was 500 kilometers per second every 3.26 million lightyears, that is, every megaparsec.

Therefore, since the curvature of the universe is essentially set by its expansion rate, the expansion rate of the universe determines how far away assuming special relativity works (which is typically as far as the expansion is negligible, so within a megaparsec or so, and also far away from any really dense objects, such as black holes or neutron stars).

The project I was running was designed to extract this information for all the galaxies in a many cubic megaparsec volume of the local Universe.

Currently, most astronomers agree that the value of the Hubble constant is about 71 kilometers per second per megaparsec a megaparsec is 3.2 million lightyears.

“As I said before, the number I came up with for the constant differs by .02 kilometers per second per megaparsec from the accepted value.”

The new measure of the Hubble constant, 74 kilometers per second per megaparsec (plus or minus 3.5), indicates that the universe is about 350 million years younger than the previous 13.7 billionyear estimate, Riess says.
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At a speed of 15,000km s1, how many megaparsec would the searation between the earth and galaxy change?

v = Hd where the velocity and distance (v, d) are related by this constant H = 64km/sec/Mpc. For for every megaparsec (1pc = 3.26 ly and the distance out to where one AU subtends a one second arc) the further out one observes that galaxies are moving an average of 64 km/sec faster.
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reesetee commented on the word megaparsec
The longest distance unit in common use, an astronomy unit equal to one million parsecs, 3.2616 million light years or 30.857 x 1018 kilometers (30.857 zettameters).
November 6, 2007