A pseudocolumn behaves like a table column, but is not actually stored in the table. You can select from pseudocolumns, but you cannot insert, update, or delete their values. A pseudocolumn is also similar to a function without arguments (refer to Functions). However, functions without arguments typically return the same value for every row in the result set, whereas pseudocolumns typically return a different value for each row.

Rownum is an example for pseudocolumns.

References

Oracle Docs