An invention is a novel device or technique.
One question that society often asks is, "What conditions lead to the development of an invention?" There are two main opposing viewpoints on this; one school of thought argues that a lack of resources leads people to invent, whereas, the other school of thought argues that only an excess of resources will result in inventions.
Castles in the air (or castles in Spain) are creative ideas which cannot be created due to more earthly considerations. The history of invention is full of such castles, as inventions aren't necessarily invented in the order that is most useful. The design of the parachute was worked out before the invention of powered flight.
See the Timeline of invention for a detailed list of inventions, listed by date of invention.