Wilting is the loss of rigidity in the non-woody above-ground parts of a plant. For example, a plant might wilt in drought conditions.
The term "wilt" is also used to describe a wide range of plant diseases. These typically affect the plant's vascular system causing non-woody parts of the plant to droop and die. Some wilts start from the roots up, others start at the extremities and work back.
The term describes the symptom and not the disease, so a range of different pathogens can be responsible and the proper course of treatment will be dependent on the pathogen.
Use this tag for questions wilting as a disease - especially if the disease is unknown. Include the plant type, and other symptoms (eg. do the leave change color). Also anything unusual such as climate variations in recent weeks.