1 (used in nouns and adjectives 用于构成名词及形容词) below 下面；在…之下: underground 地下undergrowth 下层灌木丛

2 (used in nouns 用于构成名词) lower in age, level or position （年龄、水平、地位）低于…的: the under-fives 五岁以下儿童an under-secretary 副部长

3 (used in adjectives and verbs 用于构成形容词及动词) not enough 不足: undercooked food 煮得欠熟的食物