Most aquatic and semi-aquatic animals swim by moving their bodies or tails side to side, or up and down. This wave-like motion is called undulation and it is used by reptiles, whales, and fish. A few exceptions exist: turtles and penguins are aquatic animals that swim by repeatedly moving their paddles (this movement is called oscillatory movement or flapping) Animals like seals, walruses, and plesiosaurs swim using a combination of the movements of their paddles, tails, and bodies, combining undulation and oscillation, or repeated movements. When talking about forms of swimming, it usually refers to flapping. movements.
Animals that undulate do not simply move their tails side-to-side or up and down to swim. Undulatory swimming involves movement of the whole body, because the wave-like motion passes along most of the vertebral column in order to reach the tail. Animals that swim with an undulatory movement have some features of their vertebral columns that allow them to make undulations of the appropriate size. For example, certain regions of the vertebral column may be reinforced, stiffened, elongated, or shortened, depending on the species.

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