expression, noun
pencil lead
Refers specifically to the graphite core inside a pencil.
鉛筆の芯が折れた。
The pencil lead broke.
この鉛筆の芯は柔らかい。
This pencil's lead is soft.
Refers to mechanical pencil lead, while 鉛筆の芯 is specifically for wooden pencils.
Compound of 鉛筆 (pencil) + の (possessive particle) + 芯 (core).