noun
Refers to strings made from animal intestines (catgut) or similar material, used for musical instruments, surgical sutures, or sports rackets. The term is somewhat technical.
バイオリンの弦線が切れたので、新しいのを張り替えた。
The violin string broke, so I replaced it with a new one.
昔のテニスラケットには弦線が使われていた。
Old tennis rackets used catgut strings.
弦 is a more general term for 'string' (of a musical instrument) and is much more common than 弦線, which specifically refers to the material (catgut) or the string itself in a technical context.
糸 means 'thread' or 'yarn' in general, not specifically a musical string. 弦線 is a specialized type of string.
Compound of 弦 (げん, 'string') and 線 (せん, 'line' or 'wire'), literally 'string line'. The term likely arose to distinguish catgut strings from other types of strings or wires.