Here's a link to some information on Sciatica:
http://www.spine-health.com/topics/c.../sciatica.html

I don't know the exact definition (heck, I don't know if there is one). But I've heard it used before in the diagnosis of MS (but it's usually in the brain). But then again, it is mentioned for a lot of stuff so it's hard telling. I know anytime my doctors don't know what some foreign thing is in my body it is called a lesion until proven otherwise. I'm not really familiar with those things as it can be, well, anything. It's always best to confirm what 'it' is, though. In my opinion at least.

Just what you originally described is what made me think of sciatica.

Sciatica Symptoms

The most common symptom from sciatica is pain. Most people describe a deep, severe pain that starts low on one side of the back and then shoots down the buttock and the leg with certain movements.

• The pain is usually worse with both prolonged sitting and standing. Some people describe the worst pain when trying to stand from a low sitting position, such as standing up after sitting on a toilet seat.
  
• In most people, the pain is made worse by sneezing, coughing, laughing, or a hard bowel movement. Bending backward can also make the pain worse.
  
• You may also notice a weakness in your leg or foot, along with the pain. The weakness may become so bad you can't move your foot.