Search notes:

ORA-00911: invalid character

Although the dollar sign, the underscore and numbers are permitted in identifiers, identifiers cannot start with these characters (except when quoted):

select _leading_underscore from dual;

String literals need to be quoted:

create table tq84_xyz(
  col varchar2(20)
);

insert into tq84_xyz values (#);

drop table tq84_xyz;

Special characters such as the greek μ which is commonly used to denote micro (as in microseconds) is not allowed in identifiers:

create table tq84_t (
   time_µsecs  number
);

If the μ needs to be in the column name, it can be quoted:

create table tq84_t (
  "time_µsecs" number
);

ORA-00911: invalid character

See also