Two distinct uses, distinguished by capitalization in pinyin. As a common noun zhǎngsūn it means 'eldest grandson' - historically a position of inheritance significance in Chinese patrilineal families. As a surname Zhǎngsūn it's one of the rare two-character Chinese family names; the most famous bearer is
长孙无忌 (Zhangsun Wuji, c. 594-659), brother of Empress Zhangsun and a key minister to Tang Taizong.
