什么是数据字典？请给出一个数据字典的实例

数据字典是一种文档或数据库，用于记录系统、应用程序或数据库中使用的数据元素及其相关属性的详细信息。它是一份技术文档，提供了数据的定义、结构、格式、存储方式、访问控制和其他相关信息，有助于开发人员和其他相关人员理解数据元素的含义和用途。

以下是一个数据字典的实例：

Table Name: Employee

| Field Name | Data Type | Length | Constraints | Description |
|--|
| EmployeeID | int | 4 bytes | Primary key, auto-increment | Unique identifier for employee |
| FirstName | varchar | 50 characters | Not null | First name of employee |
| LastName | varchar | 50 characters | Not null | Last name of employee |
| BirthDate | date | 3 bytes | Not null | Birth date of employee |
| HireDate | date | 3 bytes | Not null | Hire date of employee |
| Address | varchar | 100 characters | Not null | Address of employee |
| City | varchar | 50 characters | Not null | City of employee |
| State | varchar | 50 characters | Not null | State of employee |
| ZipCode | varchar | 10 characters | Not null | Zip code of employee |