What is the X in X-mas?
late 14c., "small part or division of a whole, minute portion of matter," from Latin particula "little bit or part, grain, jot," diminutive of pars (genitive partis) "part;" see part (n.). Particle physics attested from 1969. In construction, particle board (1957) is so called because it is made from chips and shavings of wood.
particle par·ti·cle (pär'tĭ-kəl)
A very small piece or part.
An elementary particle.
A subatomic particle.