To answer you question as a apache URL layout configuration via mod_rewrite (the apache module that handles URL rewriting), this could be helpful:
RewriteRule ^admin/(.*)$ /wp-admin/$1 [QSA,L]
(untested configuration directive, check the docs in case this does errors)
This will basically translate any request to /admin/… into /wp-admin/… .
If that’s the only thing you would like to know, I must admit that this is slightly off-topic for worpdress@se as this is acutally a question how to configure the apache webserver. A better place might be serverfault.
Keep in mind that this only rewrites the URLs that get requested. WordPress would know nothing about this and it does still output links to /wp-admin/ on the admin then. There is a hook for any admin URL that you might want to rewrite then as well:
return apply_filters('admin_url', $url, $path, $blog_id);
You would need to replace the domain/wp-admin/
part with domain/admin/
with a filter on your own.