trainer
n. countablen. a person who teaches people or animals how to do a specific job or skill. In British English, it is also a comfortable shoe you wear for sports or exercise.
n. a person who instructs others in a particular skill or physical activity. In British English, it refers to a soft-soled shoe designed for athletic use.
I asked my fitness trainer for a new workout plan.
The company hired a professional trainer to teach the staff how to use the new software system.
While the athlete possessed natural talent, it was the guidance of a dedicated trainer that refined his technique for the national championships.
In the footwear sense, the word is almost always used in the plural ('trainers').
I bought a new trainer for my left foot.I bought a new pair of trainers.When referring to footwear, the word is used in the plural or with 'a pair of'.