钟头 (zhōng tóu) is a colloquial word for 'hour', used mainly in everyday speech (e.g., 一个钟头). The standard, neutral term is 小时 (xiǎo shí), which is appropriate in both speech and writing. 时 (shí) as a classifier for hours is more formal and appears in fixed expressions or written contexts (e.g., 八时).