Defines a single bucket of all the documents within the search execution context. This context is defined by the indices and the document types you’re searching on, but is not influenced by the search query itself.

Example:

POST /sales/_search?size=0
{
    "query" : {
        "match" : { "type" : "t-shirt" }
    },
    "aggs" : {
        "all_products" : {
            "global" : {}, 
            "aggs" : { 
                "avg_price" : { "avg" : { "field" : "price" } }
            }
        },
        "t_shirts": { "avg" : { "field" : "price" } }
    }
}

The above aggregation demonstrates how one would compute aggregations (avg_price in this example) on all the documents in the search context, regardless of the query (in our example, it will compute the average price over all products in our catalog, not just on the "shirts").

The response for the above aggregation:

{
    ...
    "aggregations" : {
        "all_products" : {
            "doc_count" : 7, 
            "avg_price" : {
                "value" : 140.71428571428572 
            }
        },
        "t_shirts": {
            "value" : 128.33333333333334 
        }
    }
}