In many societies, it is customary to bring a gift when invited to someone's home.

customary 是形容词，来自名词 custom（习俗、惯例），加上后缀 -ary 表示"与...有关的"。customary 意思是"习惯上的、惯例的"，指按照社会或文化传统通常要做的事情。