Add definitions for VM identifiers and definitions.

index.html

@@ -1139,6 +1139,12 @@ <h2>Verification Methods</h2>
 or "proof."
           </p>
 
+          <p>
+A [=verification method=] is defined in a [=controller document=] using the
+[=map=] below and is referred to as the
+<dfn>verification method definition</dfn>:
+          </p>
+
           <dl>
             <dt><dfn class="lint-ignore">verificationMethod</dfn></dt>
             <dd>
@@ -1162,9 +1168,11 @@ <h2>Verification Methods</h2>
                 <dt>id</dt>
                 <dd>
                   <p>
-The value of the `id` property for a [=verification method=]
-MUST be a <a data-cite="INFRA#string">string</a> that
-conforms to the [[URL]] syntax.
+The value of the `id` property for a [=verification method=] MUST be a <a
+data-cite="INFRA#string">string</a> that conforms to the [[URL]] syntax. This
+value is called the <dfn>verification method identifier</dfn> and can also be
+used in a [=proof=] to refer to a specific instance of a [=verification
+method=], which is called the [=verification method definition=].
                   </p>
                 </dd>
                 <dt>type</dt>
@@ -2503,9 +2511,9 @@ <h3>Retrieve Verification Method</h3>
 
         <p>
 The following algorithm specifies how to safely retrieve a verification method,
-such as a cryptographic [=public key=], by using a [=verification method=]
-identifier. Required inputs are a <strong>[=verification method=]</strong>
-identifier (<var>vmIdentifier</var>), a
+such as a cryptographic [=public key=], by using a [=verification method
+identifier=]. Required inputs are a <strong>[=verification method
+identifier=]</strong> (<var>vmIdentifier</var>), a
 <strong>[=verification relationship=]</strong>
 (<var>verificationRelationship</var>), and a set of <strong>dereferencing
 options</strong> (<var>options</var>). A
@@ -2601,7 +2609,7 @@ <h3>Retrieve Verification Method</h3>
         </pre>
 
         <p class="note" title="Controller documents can contain references to external verification methods">
-[=Verification method=] identifiers are expressed as strings that are URLs, or
+[=Verification method identifiers=] are expressed as strings that are URLs, or
 via the `id` property, whose value is a URL. It is possible for a [=controller
 document=] to express a [=verification method=], through a [=verification
 relationship=], that exists in a place that is external to the [=controller
@@ -2632,8 +2640,8 @@ <h3>Retrieve Verification Method</h3>
 
         <p>
 In the example above, the algorithm described in this section will use the
-`https://external.example/xyz#key-789` URL value as the [=verification method=]
-identifier. The algorithm will then confirm that the [=verification method=]
+`https://external.example/xyz#key-789` URL value as the [=verification method
+identifier=]. The algorithm will then confirm that the [=verification method=]
 exists in the external [=controller document=] and that the appropriate
 relationships exist as described earlier in this section.
         </p>