It is a difficult question to answer as the names keep changing. Here is a timeline about training as a (typical) NHS doctor, and current (and previous) titles and names.
Year 1. Medical Student (Age 18 years) – training starts
Year 2. Medical Student
Year 3. Medical Student
Year 4. Medical Student
Year 5. Medical Student, qualifying as a doctor
Year 6. Foundation Year 1 (F1). Previous term = ‘House officer’, HO (or Junior House Officer, ‘JHO’)
Year 7. Foundation Year 2 (F2)
Year 8. Core or Specialty Trainee (CT1/ST1). If medical (not surgical), also called an ‘IMT’. Previous term = ‘SHO’ (Senior House Officer)
Year 9. Core or Specialty Trainee (CT2/ST2) – SHO
Year 10. Core or Specialty Trainee (CT3/ST3) – SHO
Year 11. Specialty Trainee (ST4) – ‘Registrar’. Previous term ‘SpR’ (Specialist Registrar) and before that ‘SR’ (Senior Registrar)
Year 12. Specialty Trainee (ST5) – Registrar
Year 13. Specialty Trainee (ST6) – Registrar
Year 14. Specialty Trainee (ST7) – Registrar
Year 15. Specialty Trainee (ST8) – Registrar
Year 16. Consultant (Age 34 years) – training complete.
For different types of doctor, the training is slightly different. For example, a GP does a 3 year GP training scheme in years 8-10, and then can work independently.
We have described what are junior doctors called. We hope it is clearer now.