MERGE table, do nothing when matched

For your case, no need to use the part:

when matched then update ...

( using when matched then update set a.id = a.id is accepted(Oracle doesn't hurl) but has no impact, so, such a usage is redundant, because you don't want to change anything for the matching case. )

If you wanted to change, then add

when matched then update set a.id = b.id

before when not matched then insert...

( e.g.Oracle supports when matched then update syntax. Refer the Demo below )

Go on with the following for the current case :

SQL> create table domains( id int, name varchar2(50), code varchar2(50), description varchar2(50));

SQL> insert into domains values(1,'Domain A','D.A.','This is Domain A');

SQL> merge into domains A
  using 
 (select 2 id, 'Domain A' name, 'D.A.' code, 'This is Domain A' description from domains) b
     on ( a.name = b.name )
   when not matched then insert( a.id, a.name, a.code, a.description ) 
                         values( b.id, b.name, b.code, b.description );

SQL> select * from domains;

ID  NAME        CODE    DESCRIPTION
--  --------   -----  ----------------
1   Domain A    D.A.  This is Domain A

SQL> delete domains;

SQL> insert into domains values(1,'Domain A','D.A.','This is Domain A');
-- we're deleting and inserting the same row again

SQL> merge into domains A
  using 
 (select 2 id, 'Domain B' name, 'D.B.' code, 'This is Domain B' description from domains) b
     on ( a.name = b.name )
   when not matched then insert( a.id, a.name, a.code, a.description ) 
                         values( b.id, b.name, b.code, b.description );

ID  NAME        CODE    DESCRIPTION
--  --------   -----  ----------------
1   Domain A    D.A.  This is Domain A
2   Domain B    D.B.  This is Domain B

Demo

Oracle SQL syntax supports not having any when matched then update clause.

drop table ft purge;
create table ft (c1 number, c2 varchar2(10));

drop table ld purge;
create table ld (c1 number, c2 varchar2(10));

insert into ft values (1,'a');
insert into ld values (1,'b');
insert into ld values (2,'c');
commit;

merge into ft 
using ld
on (ft.c1 = ld.c1) 
when not matched then
insert (c1,c2) values (ld.c1,ld.c2);

select * from ft;

C1  C2
--- ---
1   a
2   c

2 rows selected.