单一 (dān yī) means 'single, only' but often carries a negative connotation of being monotonous or lacking variety (e.g., a plain pattern, a boring diet). In contrast, 唯一 (wéi yī) is neutral, simply meaning 'the only one' without implying dullness. 单 (dān) is a more versatile word meaning 'single' as an adjective or 'only' as an adverb, but it's less common as a standalone adjective for describing monotony.