Citron
A source of confusion is that "citron" or similar words in French (and other languages), and English are false friends, as they refer to the lemon. Indeed, into the 16th century, the English name "citron" included the lemon and perhaps the lime as well.
Lemon tart
In the UK, lemon tart (also called tarte au citron) consists of a pastry case (often made in a fluted tart tin) containing a baked lemon custard (usually composed of eggs, sugar, lemon juice and cream). Usually recipes include blind-baking before adding the custard. Sometimes the tart is dusted with icing sugar prior to serving.