Whenever friends gather, he always talks endlessly about his stamp collecting hobby.

hobby 是名词，意思是"业余爱好、消遣活动"，指人们在闲暇时间出于兴趣而从事的活动。这个词通常指那些能带来愉悦感的非职业性活动，比如集邮、园艺、运动等。