根据官方定义, 简单来说，bean definition 就是一个 bean 的定义
A Spring IoC container manages one or more beans. These beans are created with the configuration metadata that you supply to the container (for example, in the form of XML
Within the container itself, these bean definitions are represented as BeanDefinition objects, which contain (among other information) the following metadata:
- A package-qualified class name: typically, the actual implementation class of the bean being defined.
- Bean behavioral configuration elements, which state how the bean should behave in the container (scope, lifecycle callbacks, and so forth).
- References to other beans that are needed for the bean to do its work. These references are also called collaborators or dependencies.
- Other configuration settings to set in the newly created object — for example, the size limit of the pool or the number of connections to use in a bean that manages a connection pool.
bean definition 包含：
- 全名限定，通常是实现类的全名限定
- bean 相关配置，包括 scope, lifecycle 等
- bean 的依赖
- 实例化的属性，比如 pool size 之类的