Migration has several meanings used routinely in chemistry. It can be:
(2) The movement of a bond to a new position, within the same molecular entity, is known as "bond migration".
Allylic rearrangements exemplify both of the above types of migration. For example:
RCH=CHCH2X → RCH(X)CH=CH2
(3) Movement of a substance's molecules in environments such as an electric field, or a concentration gradient, or a pressure or temperature difference.