Add glossary entry for disruption (#16292)

content/en/docs/reference/glossary/disruption.md

@@ -0,0 +1,25 @@
+---
+title: Disruption
+id: disruption
+date: 2019-09-10
+full_link: /docs/concepts/workloads/pods/disruptions/
+short_description: >
+  An event that leads to Pod(s) going out of service
+aka:
+tags:
+- fundamental
+---
+ Disruptions are events that lead to one or more
+{{< glossary_tooltip term_id="pod" text="Pods" >}} going out of service.
+A disruption has consequences for workload resources, such as
+{{< glossary_tooltip term_id="deployment" >}}, that rely on the affected
+Pods.
+
+<!--more-->
+
+If you, as cluster operator, destroy a Pod that belongs to an application,
+Kubernetes terms that a _voluntary disruption_. If a Pod goes offline
+because of a Node failure, or an outage affecting a wider failure zone,
+Kubernetes terms that an _involuntary disruption_.
+
+See [Disruptions](/docs/concepts/workloads/pods/disruptions/) for more information.