Define missing definitions for variables (#775)

* Define missing definitions for variables

During publication of v45, I found that we were missing a number of terms that we actively use in the specification. Among these are:

- variable
- external variable
- local variable

This PR defines those terms.

* Update spec/syntax.md

* Address comments

Responding to @catamorphism's comment.

This change adopts the already-problematic phrase "resolved value", which is begging for us to define it.

spec/syntax.md

@@ -97,6 +97,15 @@ Attempting to parse a _message_ that is not _valid_ will result in a _Data Model
 
 A **_<dfn>message</dfn>_** is the complete template for a specific message formatting request.
 
+A **_<dfn>variable</dfn>_** is a _name_ associated to a resolved value.
+
+An **_<dfn>external variable</dfn>_** is a _variable_ 
+whose _name_ and initial value are supplied by the caller
+to MessageFormat or available in the _formatting context_.
+Only an _external variable_ can appear as an _operand_ in an _input declaration_.
+
+A **_<dfn>local variable</dfn>_** is a _variable_ created as the result of a _local declaration_.
+
 > [!NOTE]
 > This syntax is designed to be embeddable into many different programming languages and formats.
 > As such, it avoids constructs, such as character escapes, that are specific to any given file