Definition: a smooth, tangy fruit preserve thickened with egg yolks and butter — lemon curd being the classic.
Curd is that glossy, sharp-sweet spread you get with citrus and other tart fruits. Juice, sugar, egg yolks and butter are cooked gently until thick enough to coat the back of a spoon, then it sets further as it cools. The eggs are what thicken it, so low and slow is the rule — too hot and they scramble. It’s lovely folded through cream, spooned over a cheesecake, or as a tart filling.
In practice: cook gently and stir constantly; pull it off the heat the moment it coats a spoon.
Related:blind baking,folding.