noun
replacement lead; refill; replacement wick
Refers to a consumable replacement part: pencil lead, pen cartridge, or lantern wick. The specific item is usually clear from context.
See also: 芯
シャープペンシルの替え芯を買った。
I bought replacement leads for my mechanical pencil.
ボールペンの替芯はどこに売っていますか。
Where are ballpoint pen refills sold?
古いランタンの替え
I'm looking for a replacement wick for an old lantern.
芯 is the core or lead itself; 替え芯 specifically means a spare or replacement core.
Compound of 替え (replacement) + 芯 (core/lead/wick).