后患 literally means 'later trouble' or 'future harm': 后 (hòu) means 'after' or 'later', and 患 (huàn) means 'trouble', 'disaster', or 'illness'. Together, they refer to a hidden or delayed negative consequence that arises after an initial action or event — often one that seems harmless or beneficial at first. It implies a latent danger that may not be immediately apparent but can cause serious problems down the road.

This term is commonly used in formal, cautionary, or analytical contexts — such as public health warnings, policy discussions, historical analysis, or medical advice. It carries a slightly literary or solemn tone and is rarely used in casual daily speech. You’ll often see it paired with verbs like 留下 (to leave behind), 酿成 (to brew/cause), or 消除 (to eliminate), emphasizing foresight and prevention.