延伸 (yán shēn) means 'to extend, stretch' — it emphasizes continuous extension along a line or direction, in space, time, or abstractly (e.g., a railway, influence). It differs from 延长 (yán cháng), which focuses on prolonging duration or length, and from 扩展 (kuò zhǎn), which means expanding in scope or area. 伸 (shēn) is more concrete, used for stretching body parts.