‘先兆’ literally combines 先 (xiān, meaning 'before' or 'earlier') and 兆 (zhào, meaning 'omen', 'sign', or 'portent'). Together, it refers to an early indication that something—often something significant, serious, or impending—is about to happen. It carries a slightly formal or medical tone and is commonly used in contexts where subtle, preliminary signals precede a larger event, especially in health (e.g., warning signs of illness) or natural phenomena (e.g., tremors before an earthquake).

The word implies causality or predictive value: a ‘先兆’ isn’t just coincidence—it’s a meaningful harbinger. While neutral in theory, it often appears in negative contexts (e.g., stroke or labor ‘premonitory symptoms’), though it can also describe positive developments (e.g., early signs of economic recovery). It functions exclusively as a noun and typically appears after descriptive modifiers like ‘明显的’ (obvious) or ‘轻微的’ (mild), or with verbs like ‘出现’ (to appear) or ‘有’ (to have).