A term which has been defined in at least two ways
(1) Focused training and practice in a single specialty–e.g., either haematology or oncology*, of what has traditionally been regarded–at least in the USA–as a dual-specialty–e.g., haematology-oncology**.
*Or obstetrics or gynaecology **Or OB/GYN (obstetrics and gynaecology)
(2) A subspecialty in its own right–e.g., paediatric cardiology, clinical neurophysiology
There appears to be little reason to retain this term when subspecialty is both more widely used (2,360,000 vs 166 hits for monospecialty) and understood.