from 
man bash
...
       unset [-fv] [name ...]
              For each name, remove the corresponding variable or
              function.   If  no  options are supplied, or the -v
              option is given, each name refers to a shell  vari
              able.  Read-only variables may not be unset.  If -f
              is specifed, each name refers to a shell  function,
              and the function definition is removed.  Each unset
              variable or function is removed from  the  environ
              ment passed to subsequent commands.  If any of RAN
              DOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or
              DIRSTACK are unset, they lose their special proper
              ties, even if they  are  subsequently  reset.   The
              exit status is true unless a name does not exist or
              is readonly.