- Revised `OpenAPI v2` fetching for CustomResourceDefinitions. CRDs are now aggregated lazily,
  which improves resource usage during installation of many CRDs. As a result, the first request
  to fetch the OpenAPI may be slower. ([#118808](https://github.com/kubernetes/kubernetes/pull/118808), [@Jefftree](https://github.com/Jefftree))

- The example API server has renamed its `wardle.k8s.io` API group to `wardle.example.com` ([#81670](https://github.com/kubernetes/kubernetes/pull/81670), [@liggitt](https://github.com/liggitt))
- CRDs defaulting is promoted to GA. Note: the feature gate CustomResourceDefaulting will be removed in 1.18. ([#84713](https://github.com/kubernetes/kubernetes/pull/84713), [@sttts](https://github.com/sttts))

    - [API Machinery](#api-machinery)
      - [kube-apiserver](#kube-apiserver)
      - [Dynamic Admission Control](#dynamic-admission-control)
      - [Custom Resource Definitions (CRDs)](#custom-resource-definitions-crds)
      - [Garbage Collector](#garbage-collector)
      - [Monitoring/Prometheus](#monitoringprometheus)
      - [Go Client](#go-client)
  - [External Dependencies](#external-dependencies)

- Fixed a bug that prevents kubectl to validate CRDs with schema using x-kubernetes-preserve-unknown-fields on object fields.

* Fixes incompatibility with custom scheduler extender configurations specifying `bindVerb` ([#65424](https://github.com/kubernetes/kubernetes/pull/65424), [@liggitt](https://github.com/liggitt))
* Using `kubectl describe` on CRDs that use underscores will be prettier. ([#65391](https://github.com/kubernetes/kubernetes/pull/65391), [@smarterclayton](https://github.com/smarterclayton))

* 'cj' has been added as a shortname for CronJobs, as in `kubectl get cj` ([#59499](https://github.com/kubernetes/kubernetes/pull/59499), [@soltysh](https://github.com/soltysh))

* `crds` has been added as a shortname for CustomResourceDefinition, as in `kubectl get crds` ([#59061](https://github.com/kubernetes/kubernetes/pull/59061), [@nikhita](https://github.com/nikhita))

  For more information, see [#53006](https://github.com/kubernetes/kubernetes/issues/53006).

*   Custom Resource Definitions (CRDs) are not properly deleted under certain conditions. This issue will be fixed in a future release.

  For more information, see [#56348](https://github.com/kubernetes/kubernetes/issues/56348).

* Added Establishing Controller on CRDs to avoid race between Established condition and CRs actually served. In HA setups, the Established condition is delayed by 5 seconds. ([#63068](https://github.com/kubernetes/kubernetes/pull/63068), [@xmudrii](https://github.com/xmudrii))

- CRD supports multi-version Schema, Subresources and AdditionalPrintColumns (NOTE that CRDs created prior to 1.13 populated the top-level additionalPrinterColumns field by default. To apply an updated that changes to per-version additionalPrinterColumns, the top-level additionalPrinterColumns field must be explicitly set to null). ([#702...

#### Extensibility

- The community has been asking for continuing support of extensibility, so this cycle features more work around CRDs and API Machinery. Most of the enhancements in this cycle were from SIG API Machinery and related areas.

### Extensibility around core Kubernetes APIs

#### CustomResourceDefinitions Pruning