n.
1.the movement of persons from one country or locality to another
2.a group of people migrating together (especially in some given time period)
3.(chemistry) the nonrandom movement of an atom or radical from one place to another within a molecule
4.the periodic passage of groups of animals (especially birds or fishes) from one region to another for feeding or breeding
Idiom of the Day
out of courtesy
in order to be polite to someone
We phoned our customers out of courtesy to tell them about the late arrival of the products.