随机 (suí jī) means 'random' in a statistical or procedural sense — it describes something done without pattern or bias, like random sampling or random generation. It differs from 随便 (suí biàn 'casual, as one pleases'), which implies a lack of care or effort, and from 乱 (luàn 'chaotic, indiscriminately'), which suggests disorder or messiness. 随机 is neutral and technical; 偶然 (ǒu rán) means 'by chance' and focuses on an unexpected event rather than a method.