提炼：

It refers to the top-level, or root object that is serialized into JSON and strored in Elasticsearch under a unique ID.
文档是指 Elasticseach 中存储的顶级JSON对象（或者根对象），拥有一个唯一的 ID。

原文：

在大多数应用中，多数实体或对象可以被序列化为包含键值对的 JSON 对象。 一个键可以是一个字段或者字段的名称，一个值可以是一个字符串，一个数字，一个布尔值，另一个对象，一些数组值，或一些其他特殊类型注入表示日期的字符串，或代表一个地理位置的对象:
Most entities or objects in most applications can be serialized into a JSON object, with keys and values. A key is the name of a field or property, and a value can be a string, a number, a Boolean, another object, an array of values, or some other specialized type such as a string representing a data or an object representing a geolocation:

{ “name”: “John Smith”, “age”: 42, “confirmed” true, “join_date”: “2014-06-01”, “home”: { “lat”: 51.5, “lon”: 0.1 }, “accounts”: [ { “type”: “facebook”, “id”: “johnsmith” },{ “type”: “twitter”, “id”:”johnsmith” } ] }

通常情况下，我们使用的术语 对象 和 文档 是可以互相替换的。 不过， 有一个区别：一个对象仅仅是类似于 hash、 hashmap、 字段或者关联数组的 JSON 对象， 对象中也可以嵌套其它的对象。对象可能包含了另外一些对象。在 Elasticsearch 中，术语 文档有着特定的含义。它是指最顶层或者根对象， 这个根对象被序列化成 JSON 并存储到 Elasticsearch 中， 指定了唯一 ID。
Often, we use the terms object and document interchangeably. However, there is a distinction. An object is just a JSON object — similar to what is known as a hash, hashmap, dictionary, or associative array. Objects may contain other objects. In Elasticsearch, the term document has a specific meaning. It refers to the top-level, or root object that is serialized into JSON and strored in Elasticsearch under a unique ID.