Refers to a person who regularly writes letters or articles for newspapers, magazines, or similar publications. Often implies an amateur or non-staff contributor.
He often contributes opinions as a correspondent for the local newspaper.
Compound of 投書 (tōsho, 'letter to the editor; contribution') + 家 (ka, 'person; specialist'). The exact historical derivation is uncertain, but the word follows a common pattern for naming practitioners of an activity.