What is a Succulent Plant?

What is a Succulent Plant?
Succulent (Latin: succos = juice, sap) plants from more than 60 families and 300 genera have evolved special water-storage tissues in thickened or swollen leaves, stems or roots as an adaptation to the arid climates of deserts and semi-deserts. Many of these habitats are associated with high day-time temperatures and special mechanisms have evolved to collect and conserve the limited moisture that is available, sometimes only from dews, mists and fogs. By making the most of scarce available moisture, succulents can survive in habitats that are far too dry for most other plants.
 
Leaf Succulents: Leaves are almost entirely composed of water storage cells covered by a thin layer of photosynthetic tissue.
Examples: Aloe, Haworthia, Lithops, Sempervivum.  
Stem Succulents: Fleshy stems contain water storage cells overlaid by photosynthetic tissue. Leaves are almost or entirely absent, reducing surface area to prevent evaporative loss of water.
Examples: most cacti, Euphorbia obesa, Stapelia.  
Root Succulents: Swollen fleshy roots store water underground away from the heat of the sun and hungry animals. Stems and leaves are often deciduous and shed during prolonged dry seasons.
Examples: Calibanus hookeri, Fockea capensis, Pterocactus kunzei, Peniocereus viperinus.  
Caudiciform Succulents store water in both roots and swollen stems, with either deciduous or long-lived fleshy succulent leaves. Examples: Ceraria pygmaea, Tylecodon, Cyphostemma juttae.  
Halophyte Succulents are able to survive in salty desert or marine environments by biochemical resistance to salt, sequestering salt from the cytoplasm in special vacuoles or by excreting salt. Examples: Salicornia, Sarcocornia.  
The above types may occur in combination, with more than one organ used to store water.