Sciatica means pain caused by irritation or compression of a nerve root in the lower back, producing symptoms along the path of the sciatic nerve. The distinguishing feature is that the leg symptoms usually dominate over the back symptoms, follow a recognisable band down the leg, and often come with pins and needles, numbness or weakness rather than just ache.
A great deal of what gets called sciatica is not. Referred pain from the lower back, the sacroiliac joint or the gluteal muscles can produce buttock and thigh pain that feels similar but stops short of the knee, has no neurological symptoms, and behaves differently. The distinction matters because the expected timeline and the treatment emphasis are not the same — which is the first thing the assessment sorts out.