The primary ingredient in a lemon meringue pie is lemon, specifically the juice and zest, which provide the tart and refreshing flavor characteristic of this dessert. The filling combines lemon juice, sugar, egg yolks, and cornstarch or flour, creating a creamy, tangy custard that contrasts beautifully with the sweetness of the meringue topping. The meringue, made from whipped egg whites and sugar, is often lightly browned in the oven, adding a crunchy texture. Lemon meringue pie is a delightful dessert that balances tartness with sweetness, making it a popular choice among many dessert lovers.