Although the term "reserved" does indeed appear in the Java Language Specification, Java does not have a formal definition of the
word "reserved". If you try to look it up in the index you will find that it isn't there. The JLS provides a list of keywords, but no similar list of reserved words.
The Java Language Specification makes statements such as the following.
The keywords const and goto are reserved, even though they are not currently used. This may allow a Java compiler to produce better error messages if these C++ keywords incorrectly appear in programs.
The JLS appears to use the word "reserved" to describe a characteristic of a keyword, but the JLS does not formally define a "reserved" word.