Extract definition of response key from #1039

Author: benjie

Diff for: spec/Section 2 -- Language.md

@@ -466,8 +466,9 @@ These two operations are semantically identical:
 
 Alias : Name :
 
-By default a field's response key in the response object will use that field's
-name. However, you can define a different response key by specifying an alias.
+:: A _response key_ is the key in the response object which correlates with a
+field's result. By default the response key will use the field's name; however,
+you can define a different response key by specifying an alias.
 
 In this example, we can fetch two profile pictures of different sizes and ensure
 the resulting response object will not have duplicate keys: