不良 (bù liáng) is a formal, written adjective meaning 'bad' or 'harmful', often used in fixed collocations like 不良行为 (bad behavior) or 不良影响 (harmful effects). It sounds more neutral and official than the everyday word 坏 (huài), which covers a broader range of meanings (bad, broken, spoiled). It also differs from 有害 (yǒu hài 'harmful'), which is more direct and describes concrete damage or toxicity.