The term 'Substitute Doctor' can be defined as a Locum, which is a five-letter word. A Locum is a doctor who temporarily takes the place of another doctor, usually for a short period of time. This type of doctor is often used to cover for a doctor who is on vacation or has left a practice. They are also used to fill in for doctors who are ill or have other commitments. Locums are usually employed by hospitals or medical practices and are expected to provide the same level of care as the regular doctor.