及时 (jí shí) means 'timely' or 'promptly' — it emphasizes doing something at the right moment, often to prevent a problem. Unlike 准时 (zhǔn shí) and 按时 (àn shí), which mean 'on time' according to a schedule, 及时 focuses on the appropriateness of the timing. As an adverb, it is similar to 立刻 (lì kè) but with a nuance of being just in time.