“The broad term succulent describes plants from many plant families that store water against times of drought in specialized tissues, whether in fleshy leaves (such as those of a jade plant), stems (such as most cacti), or a stem-root structure known as a caudex . All cacti are succulents, as are the desert dwellers agaves, aloes, and many euphorbias…. In fact, about 10,000 plant species in 30 families are considered succulents."
From Ray Rogers, Crazy About Cacti and Succulents. 2006