Liquefaction is the transformation of a soil from a solid to a liquid state, usually (but not always) as a result of increased water content.
When water is added gradually to a dry soil, the material first becomes plastic, or moldable. If enough water is added, the particles lose contact with one another and the material turns into a loose slurry, losing its shear strength in the process. The point at which this transition occurs, is called the liquid limit, varies from one soil to another. Some materials, particularly some clay-bearing soils, have very high liquid limits and may remain plastic over a broad range of water contents.