The name of the episode, "Arcadia", comes from the Latin "Et in Arcadia ego", literally "I, too, have lived in Arcadia" - a Latin way of saying "I, too, have known better times". Arcadia was a paradise in Latin culture and belief.
Tulpa: Translated as "magical emanation", conjured thing" and "phantom" is a concept in mysticism of a being or object which is created through sheer spiritual or mental discipline alone. It is defined in Indian Buddhist texts as any unreal, illusory or mind created apparition.
A whirligig is an object that are most commonly powered by the wind which spins or whirls, or has at least one member that spins or whirls. Also known as pinwheels, buzzers, comic weathervanes, gee-haws, spinners, whirligigs, whirlijig, whirlyjig, whirlybird, or plain whirly.