For instance there's following configuration in httpd.conf:

ProxyPass app   http://somehost:someport/App_1   
ProxyPassReverse app   http://somehost:someport/App_1   

Now I ought to use absolute web addresses to forward demands from "/myapp" to "/application":

<Location /myapp >   
   ProxyPass http://localhost:8080/app
   ProxyPassReverse http://localhost:8080/app
</Location>

Can you really use relative path in ProxyPass/ProxyPassReverse?

<Location /myapp >  
   ProxyPass /app  
   ProxyPassReverse /app  
</Location>  

No.

The apache docs about ProxyPass states the prospective should be a URL. By trying to place something which is not a URL (like /app), you receive the next error:

ProxyPass URL should be absolute!

You need to consider mod_rewrite rather. It may rewrite demands around the serverside without redirecting the browser. Giving your /myapp -> /application example, some simple rules similar to this would suffice:

RewriteRule ^/myapp /app [L]