Wyrd is a Changeling term for the invocation of magical or otherwise chimerical effects that are able to affect the mortal, mundane world. The Wyrd enables these effects to affect mundane people, places, or things with their supernatural abilities.
Some cantrips known to Changelings may be used to affect humans or ordinary objects by "calling on the Wyrd", using more of the Changeling's glamour to achieve an effect which clearly contradicts the normal rules of the mundane world. Other cantrips are specifically designed to affect such targets, and do not require additional glamour. Such effects are noticed by even unenchanted mortals, though they are usually forgotten as the Mists do their work.
Chimera and other denizens of the Dreaming may also learn to become Wyrd, either through Redes or other means, manifesting themselves in the normal world. This is typically a very temporary state, as such creatures are highly susceptible to banality.