辛勤 (xīn qín) is a formal, literary adjective meaning 'hardworking, industrious', often used to describe sustained effort in work or labor (e.g., 辛勤劳动). It has a more solemn, commendatory tone than the more general 勤奋 (diligent) and is less colloquial than 勤快. Unlike 勤劳, which focuses on habitual hard work in manual labor, 辛勤 emphasizes dedicated effort and can function as an adverb (辛勤地).