Sciatica
Sciatica is a general term for pain along the sciatic nerve which runs from the low back down the back of the leg. Typically the pain is present in only one of the lower extremities and stems from pressure or irritation to one or more nerve roots, the nerve roots from lumbar 4 (L4), L5, and Sacral 1 (S1) collectively form the sciatic nerve.
There are many causes of sciatica including disc herniation, stenosis, muscle spasms, etc.
Symptoms generated by the nerve itself are considered "radicular" and may include pain, numbness, weakness, and tingling.
Sciatica may also result from Somatic referred symptoms (i.e. you feel it in your leg when the problem is actually a muscle in your back).