需(xū) - to need, to require in Mandarin

需 (xū) is the bound morpheme behind the ordinary word 需要 (xū yào, to need). On its own it means the same thing, but it belongs to written and formal registers: notices, contracts, official signs and set phrases such as 如需 (if required) and 无需 (no need to).

That register is the whole point of learning it separately. 需 lets a sentence stay short and clipped, which is why you meet it on a form long before you meet it in conversation. Compare 本店需提前预约 on a shop door with the spoken 我们店需要提前预约: same meaning, very different clothes.

The trap is using it where you should use 需要. In speech, a bare 需 sounds stilted or plain wrong, and 我需一杯水 is not something anyone says. Keep 需 for writing and fixed expressions, and keep 需要 for talking.