不然 (bù rán) is a conjunction meaning 'otherwise, or else', commonly used in spoken Chinese. It is more colloquial than 否则 (fǒu zé), which is preferred in formal writing. 要不 (yào bù) is also colloquial and often interchangeable, but 不然 can also be used sentence-finally as a tag question ('不然呢？' — 'how about it?').