[[mapping-boost-field]]
=== `_boost`

Boosting is the process of enhancing the relevancy of a document or
field. Field level mapping allows to define explicit boost level on a
specific field. The boost field mapping (applied on the
<<mapping-root-object-type,root object>>) allows
to define a boost field mapping where *its content will control the
boost level of the document*. For example, consider the following
mapping:

[source,js]
--------------------------------------------------
{
    "tweet" : {
        "_boost" : {"name" : "my_boost", "null_value" : 1.0}
    }
}
--------------------------------------------------

The above mapping defines mapping for a field named `my_boost`. If the
`my_boost` field exists within the JSON document indexed, its value will
control the boost level of the document indexed. For example, the
following JSON document will be indexed with a boost value of `2.2`:

[source,js]
--------------------------------------------------
{
    "my_boost" : 2.2,
    "message" : "This is a tweet!"
}
--------------------------------------------------
