Rubyの予約語

BEGIN class ensure nil self when
END def false not super while
alias defined? for or then yield
and do if redo true __LINE__
begin else in rescue undef __FILE__
break elsif module retry unless __ENCODING__
case end next return until

http://doc.ruby-lang.org/ja/1.9.3/doc/spec=2flexical.html

※Railsの予約語も参照のこと