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 }
    }
  }
}