elasticsearch/docs/reference/mapping/fields/source-field.asciidoc

[[mapping-source-field]]
=== `_source`

The `_source` field is an automatically generated field that stores the
actual JSON that was used as the indexed document. It is not indexed
(searchable), just stored. When executing "fetch" requests, like
<<docs-get,get>> or
<<search-search,search>>, the `_source` field is
returned by default.

Many APIs may use the `_source` field. For example, the
<<docs-update,Update API>>. To minimize the storage cost of
`_source`, set `index.codec: best_compression` in index settings.