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

[[mapping-id-field]]
=== `_id`

Each document indexed is associated with an id and a type. The `_id`
field allows accessing only the id of a document.

Note, even though the `_id` is not indexed, all the APIs still work
(since they work with the `_uid` field), as well as fetching by ids
using `term`, `terms` or `prefix` queries/filters (including the
specific `ids` query/filter).