When talking about study areas or degrees, 'specialism' relates to someone's main subject. She is completing a specialism in environmental science.
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Professional Expertise
In a work context, 'specialism' can refer to the specific skills or knowledge someone is known for. His specialism in graphic design is widely acknowledged.
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Personal Passion
Specialism may also imply a deep personal interest, beyond professional or academic requirements. Her specialism in ancient languages comes from a deep love for history.