The basic representation unit in most rule-based expert systems. Similar to variables in traditional data processing, but different in that they may own properties, and do not necessarily represent memory storage locations.在大多数基于规则的专家系统中的基本表示单元。它类似于传统数据处理中的变量，但有不同之处，属性可具有性质，而不一定代表记忆存储单元。
