"Sagging" is when the bow and stern of the ship are both supported by a wave crest and midships (the middle of the ship) is in the wave trough - thus unsupported. This causes the middle of the ship to "sag".

"Hogging" is just the opposite, the middle of the ship is supported by the wave crest while the bow and stern are unsupported over the adjacent troughs.

Hogging and sagging are typically approximated by predefined displacements. Wave motion is typically approximated by applied accelerations (G factors).