also: ドータカード
noun
daughter card; daughterboard
A secondary circuit board that attaches to a motherboard or main board to add functions. Technical computing term.
このマザーボードにはドーターカードを増設できるスロットがある。
This motherboard has a slot where you can add a daughter card.
拡張カード is a general term for any expansion card, while ドーターカード specifically refers to a smaller board that plugs into a main board, often parallel to it.
Borrowed from English 'daughter card', a term used in computing for a secondary circuit board.