A shape that can be repeated to cover a plane with a nonrepeating pattern.
A genius; a very smart person.
One mole of photons, regardless of frequency, as used to measure irradiance.
Something apparently seen, heard, or sensed, but having no physical reality; an image that appears only in the mind; an illusion or delusion.
A ghost or apparition.
A placeholder for a pair of players when there are an odd number of pairs playing.
A test object. A test phantom is an object that reproduces the characteristics of human tissue.
Illusive.
Fictitious or nonexistent.