Defines a single bucket of all the documents in the current document set context that match a specified filter. Often this will be used to narrow down the current aggregation context to a specific set of documents.

Example:

POST /sales/_search?size=0
{
    "aggs" : {
        "t_shirts" : {
            "filter" : { "term": { "type": "t-shirt" } },
            "aggs" : {
                "avg_price" : { "avg" : { "field" : "price" } }
            }
        }
    }
}

In the above example, we calculate the average price of all the products that are of type t-shirt.

Response:

{
    ...
    "aggregations" : {
        "t_shirts" : {
            "doc_count" : 3,
            "avg_price" : { "value" : 128.33333333333334 }
        }
    }
}