Sciatica is the name given to any sort of pain that is caused by irritation or compression of the sciatic nerve.
The sciatic nerve is the longest nerve in the body. It runs from the back of the pelvis, through the buttocks, and all the way down both legs, ending at the feet.
When the sciatic nerve is compressed or irritated, it can cause pain, numbness and a tingling sensation that radiates from the lower back and travels down one of the legs to your foot and toes.
The pain can range from being mild to very painful, and may be made worse by sneezing, coughing, or sitting for a long period of time.
Some people with sciatica may also experience muscle weakness in the affected leg.
While people with sciatica can also have general back pain, the pain associated with sciatica usually affects the buttocks and legs much more than the back.
Source NHS Choices July 2016