1. n. 
The movement of hydrocarbons from their source into reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system.
See: accumulation, basin, caprock, critical moment, generation, migrate, petroleum system, preservation, primary migration, prospect, reservoir, seal, secondary migration, sedimentary basin, source rock, spill point, trap
2. n. 
A step in seismic processing in which reflections in seismic data are moved to their correct locations in the x-y-time space of seismic data, including two-way traveltime and position relative to shotpoints. Migration improves seismic interpretation and mapping because the locations of geological structures, especially faults, are more accurate in migrated seismic data. Proper migration collapses diffractions from secondary sources such as reflector terminations against faults and corrects bow ties to form synclines. There are numerous methods of migration, such as dip moveout (DMO), frequency domain, ray-trace and wave-equation migration.
See: bow tie, depth migration, depth section, diffraction, dip moveout, frequency domain, Kirchhoff migration, migrate, processing, reflection, resolution, shotpoint, sideswipe, smile, three-dimensional seismic data, time migration, two-way traveltime, velocity analysis