migration

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or an instance of migrating.
• noun A group migrating together.
• noun The movement of one atom or more from one position to another within a molecule.
• noun The movement of ions between electrodes during electrolysis.

from The Century Dictionary.

• noun In phytogeography, the movement of plants from one area into another. According to F. E. Clements this is properly a narrower term than invasion. See invasion, 4.
• noun The act of migrating; change of residence or habitat; removal or transit from one locality or latitude to another, especially at a distance.
• noun A number of animals migrating together; the total of the individuals or species which perform any particular migration; also, the time or period occupied in migrating Change of place; removal.
• noun Residence in a foreign country; banishment.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of migrating.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An instance moving a place to live to another place for a while.
• noun Seasonal moving for animals, birds or fishes to breed or find a new home.
• noun computing Instance of changing a platform from an environment to another one.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the periodic passage of groups of animals (especially birds or fishes) from one region to another for feeding or breeding
• noun a group of people migrating together (especially in some given time period)
• noun the movement of persons from one country or locality to another
• noun (chemistry) the nonrandom movement of an atom or radical from one place to another within a molecule