Straight lines in the original object remain straight. Parallel lines remain parallel. Circles deform to ellipses. Note that this definition rules out folding, since an originally straight layer has to remain straight. How real geology behaves.
Deformation varies from place to place.
Lines may bend and do not necessarily remain parallel. Terms for Behavior of Materials Elastic Material deforms under stress but returns to its original size and shape when the stress is released. There is no permanent deformation. Some elastic strain, like in a rubber band, can be large, but in rocks it is usually small enough to be considered infinitesimal. Brittle Material deforms by fracturing.