A general definition of succulents is that they are drought resistant plants in which the leaves, stem or roots have become more than usually fleshy by the development of water-storing tissue.
The storage of water often gives succulent plants a more swollen or fleshy appearance than other plants, a characteristic known as succulence. In addition to succulence, succulent plants variously have other water-saving features, (CAM) to minimise water loss:
compact, reduced, cushion-like, columnar, or spherical growth form
ribs enabling rapid increases in plant volume and decreasing surface area exposed to the sun
waxy, hairy, or spiny outer surface to create a humid micro-habitat around the plant, which reduces air movement near the surface of the plant, and thereby reduces water loss and creates shade
roots very near the surface of the soil, so they are able to take up moisture from very small showers or even from heavy dew.
ability to remain plump and full of water even with high internal temperatures (e.g., 52 °C or 126 °F) , retains water abundantly!