To manual therapists and doctors, sciatica simply means a radiating pain down the leg NOT caused by the structures of the hip, knee or foot.
Strictly speaking, sciatica means pain or aches, not other symptoms such as numbness.
A sore nerve root exiting the spinal canal in the small of the back can cause radiating pain down the back side or front of the leg. This may, but not always, also cause numbness, pins and needles, and weakness in the leg.
Although that's the most common cause of sciatica, there are others. For example, muscles of the low back or buttocks can radiate pain down the leg. Another example, the joint between the pelvis and the sacrum is also considered an occasional cause of sciatica.
True sciatica is simply pain in the leg, often caused by a trapped and sore nerve as it exits the low back.