A particular value for a variable that is assigned automatically by an operating system and remains in effect unless canceled or overridden by the operator. 缺省，默认某一变量的特定值，由操作系统自动指定并持续有效，除非操作者取消它或使它无效
Establishing a value for a variable or symbol or establishing what the variable represents. 为变量或符号确定一个值，或者确定此变量代表的内容。
A variable that is given a constant value for a specified application and that may denote the application. 一种为专用应用程序而给定一个常数值的变量，而且它可以表示该应用程序。
