To pass from one region or climate to another, usually periodically, for feeding or breeding, or to find a region or environment that is more conducive for their survival.
Most birds migrate cyclically, flying south in the winter and then back north in warmer weather. Many migrating birds travel in large V-shaped formations in the air. Monarch butterflies also migrate, passing into one area for a few days, weighing down the trees and stunning residents with their beauty, before they move on.
Fish migrate quite often, and mammals, particularly herd animals, also migrate from place to place.
Humans migrate, too. People may migrate individually, with their families, or with larger groups. Migration can involve moving from one country to another or even within a single country. Have you ever moved to another city or across the country for a job or school? Congratulations -- you migrated. People can also migrate because of a natural disaster or a civil disturbance, like a war or famine, or because they're being persecuted.