Definition: plain flour with raising agent already blended in, for an even, reliable rise.
Self-raising flour is simply plain (all-purpose) flour with baking powder — and usually a little salt — mixed through, so the rise is built in and evenly distributed. It’s the backbone of British sponges and quick bakes like scones and cobbler toppings. No self-raising in the cupboard? Make your own: add about 2 teaspoons of baking powder per 150g of plain flour and whisk it through well.
In practice: whisk it before using so the raising agent is evenly spread; don’t confuse it with plain flour in a recipe, or your bake won’t rise.