An apport is any solid, liquid or gaseous object which appears out of thin air, usually at a seance (although something like a rain of frogs could also count). They might also do things like appear to pass through solid objects or walls, or move around on their own. They are sometimes warm to touch.
Obviously, apports are good for fraudulent mediums. If business is slow and one's powers are in doubt, magically producing arcane objects (in reality, using sleight of hand or strings) will restore the customers' faith. Houdini's stage show included demonstrating medium's tricks with no magical powers whatsoever. However, there have allegedly been mediums who have managed to produce apports when no fraud was detected.
A common apport (for mediums) was ectoplasm, a kind of unearthly goo. The most bizarre apport was probably on June 3, 1871 when, according to 'The Unexplained' (Fraser Stewart Book Wholesale), the Victorian medium Mrs. Guppy was allegedly transported to a nearby seance.
Disclaimer: Just because I node it doesn't mean I believe in it.