Page "Diode" Paragraph 33

: These are actually a JFET with the gate shorted to the source, and function like a two-terminal current-limiter analog to the Zener diode, which is limiting voltage.

They allow a current through them to rise to a certain value, and then level off at a specific value.

Also called CLDs, constant-current diodes, diode-connected transistors, or current-regulating diodes.