specialist-n; 3 Senses

Sense Number 1: a medical doctor highly trained in a specific area of medicine

Commentary: SPECIALIST[+person][+doctor][+medicine][+branch][+specific]
NOTE: always implies a specialty in some field of medicine.
NOTE: GPs are considered to be professionals, experts, but not specialists (so a bit different from Sense 2)

He is seeing one of the best heart specialists in Houston.
The child should be examined by an ear specialist.
We could not find a specialist in this particular psychiatric disorder.

WordNet 3.0 Sense Numbers: 2

Sense Number 2: anyone considered to be an expert in a narrow field

Commentary: SPECIALIST[+person][+expert][+knowledge][+field][+narrow]
NOTE: implies deep knowledge on some topic, usually a narrow topic.

They detectives brought in their fingerprint specialist.
Mary has become a specialist in 18th century Dutch blue ceramics.
We talked to a tea specialist who selected some rare Jasmine varieties for us.

WordNet 3.0 Sense Numbers: 1

Sense Number 3: one of four enlisted ranks in the U.S. Army

Commentary: SPECIALIST[+role][+rank][+military][+enlisted][+army][+United_States]

He became a specialist 4, which is the rank of a corporal.
A sergeant first class is also a specialist 7, and does administrative work but does not exercise command.

WordNet 0.0 Sense Numbers: 2