Create definition of play button and update "Audio element has text alternative" plus input rules (e7aa44, 2eb176, afb423) (#1838)

* create definition of play button

* Update audio-transcript-2eb176.md

Author: carlosapaduarte

_rules/audio-as-media-alternative-afb423.md

@@ -25,9 +25,7 @@ acknowledgments:
 This rule applies to every [non-streaming](#non-streaming-media-element) `audio` element that is:
 
 - playing; or,
-- has a "play button" that is [visible][] and [included in the accessibility tree][].
-
-**Note:** A play button is an interactive element that when activated, plays the audio.
+- has a [play button][] that is [visible][] and [included in the accessibility tree][].
 
 ## Expectation 1
 
@@ -191,4 +189,5 @@ This `audio` element describes some of the text on the same page. The text on th
 ```
 
 [included in the accessibility tree]: #included-in-the-accessibility-tree 'Definition of included in the accessibility tree'
+[play button]: #play-button 'Definition of play button'
 [visible]: #visible 'Definition of visible'

_rules/audio-text-alternative-e7aa44.md

@@ -35,9 +35,7 @@ acknowledgments:
 This rule applies to any [non-streaming](#non-streaming-media-element) `audio` element for which at least one of the following is true:
 
 - has an `autoplay` [attribute value][] of true;  or,
-- has a "play button" that is [visible][] and [included in the accessibility tree](#included-in-the-accessibility-tree).
-
-**Note:** A play button is an interactive element that when activated, plays the audio.
+- has a [play button][] that is [visible][] and [included in the accessibility tree](#included-in-the-accessibility-tree).
 
 ## Expectation
 
@@ -203,4 +201,5 @@ This `a` element links to an audio file, but there is no `audio` element on this
 ```
 
 [attribute value]: #attribute-value 'Definition of Attribute Value'
+[play button]: #play-button 'Definition of play button'
 [visible]: #visible 'Definition of visible'

_rules/audio-transcript-2eb176.md

@@ -25,9 +25,7 @@ acknowledgments:
 This rule applies to every [non-streaming](#non-streaming-media-element) `audio` element that is:
 
 - playing; or,
-- has a "play button" that is [visible][] and [included in the accessibility tree][].
-
-**Note:** A play button is an interactive element that when activated, plays the audio.
+- has a [play button][] that is [visible][] and [included in the accessibility tree][].
 
 ## Expectation
 
@@ -197,4 +195,5 @@ This `audio` element has hidden native player controls.
 ```
 
 [included in the accessibility tree]: #included-in-the-accessibility-tree 'Definition of included in the accessibility tree'
+[play button]: #play-button 'Definition of play button'
 [visible]: #visible 'Definition of visible'

pages/glossary/play-button.md

@@ -0,0 +1,12 @@
+---
+title: Play button
+key: play-button
+unambiguous: true
+objective: false
+---
+
+A play button is an [instrument][] that when [activated][] plays a [media resource][].
+
+[activated]: https://html.spec.whatwg.org/#activation
+[instrument]: #instrument-to-achieve-an-objective
+[media resource]: https://html.spec.whatwg.org/multipage/media.html#media-resource