立即 means 'immediately' or 'right away' and functions strictly as an adverb modifying verbs to indicate that an action happens without any delay. The character 立 (lì) literally means 'to stand' but in this compound conveys the sense of 'standing upright — i.e., instantaneously present'; 即 (jí) means 'at once' or 'just now', reinforcing urgency. Together, they form a fixed, unchangeable two-character adverb commonly used in both spoken and written Chinese — especially in formal instructions, warnings, news reports, or polite requests.

This word is stronger and more urgent than similar terms like 马上 (mǎshàng) or 很快 (hěn kuài), and it often implies necessity or obligation — for example, in safety notices or official directives. It typically appears before the verb it modifies and cannot be split, reduplicated, or used attributively (e.g., you cannot say *立即的反应). While common in Standard Mandarin, it sounds slightly formal in casual conversation, where speakers might prefer 马上 or 赶紧.