机 (jī) is a bound form (b.f.) — a morpheme that cannot stand alone as a word but appears as a root in many common compound words. Its most frequent meanings include 'machine' (飞机, 机器), 'opportunity' (机会, 机遇), and 'crucial point' (关键). It also appears in specialized terms like 有机 ('organic') and 机关 ('mechanism/organization'). Because it is a bound form, you will never see it used independently in modern Chinese; it always combines with another morpheme.