Chronic spontaneous urticaria (CSU), often called chronic idiopathic urticaria (CIU), is the name for a certain kind of skin condition. The name of the condition itself tells you about what it is:
Chronic means something that lasts for a long time, or something that happens again and again.
Spontaneous/ Idiopathic means something that happens spontaneously and has no known cause.
Urticaria refers to raised hives (wheals) that usually have a red ring around them. These hives appear and disappear in less than 24 hours, and are often associated with itching or burning sensations.