The special or peculiar mark or characteristic of a person or thing; that for which a person is specially distinguished; an object of special attention; a special occupation or object of attention; a specialty
{i} distinguishing mark, special quality; unique ability; field in which one specializes, area of expertise
Someone's speciality is a particular type of work that they do most or do best, or a subject that they know a lot about. My father was a historian of repute. His speciality was the history of Germany