即(jí) - namely, that is; at once in Mandarin

即 (jí) does two jobs, both of them written. As a linking word it means namely or in other words, equating what came before with what comes after: 汉语即中文. As an adverb it means at once, straight away, the formal cousin of 就.

You will meet it far more often inside compounds than on its own: 即使 (even if), 即将 (about to), 立即 (immediately), 即刻 (this instant). Recognising the character in those is worth more to a beginner than producing the bare 即 in a sentence.

The trap is register. 即 belongs to notices, contracts, subtitles and the news, not to conversation. A learner who says 我即来 instead of 我马上来 sounds like a legal document walking about. Read it everywhere, and say 就 or 马上 instead.