Also known as cooked or stirred custard or custard sauce. Hot custard is usually served with puddings - it is almost always served with the traditional Christmas Pudding, usually with the addition of brandy.
Custard powder is hard to find outside of Australia. Bird's custard powder is available in specialty stores and "gourmet" sections of big groceries in the States (DiLaurenti in Seattle carries it, for example).
Some recipes suggest substituting non-instant vanilla pudding mix or pastry cream powder for custard powder. You may want to add a bit of cornstarch if using these.