Sciatica is used a term commonly used to describe pain down one leg. It usually occurs from the irritation of the large sciatic nerve arising from the lumbar spine. Often a disc bulge in the lumbar spine can protrude and touch the nerve resulting in the constant, nerve like pain down the leg. True sciatica is described as pain down, past the knee and related to the area of the disc bulge that is touching the nerve. Sometimes there is also associated loss of sensation in the leg and weakness in the muscles.
The most common cause of pain down the leg, which tends to radiate in the buttock and down the back of the leg is related to a large muscle in the buttock called the piriformis. This muscle referral pain referral patter can mimic sciatic pain and sometimes (in 0.7% of the population) the sciatic nerve runs directly through this muscle. So when it becomes tight and spasms, it can cause the sciatic pain.