The term 'PARTICLE HYPOTHETICAL' can be broken down into two words, 'PARTICLE' and 'HYPOTHETICAL'. The word 'PARTICLE' is used to describe a small piece of matter that is too small to be seen with the naked eye. The word 'HYPOTHETICAL' means something that is not proven or accepted as fact, but is suggested as a possible explanation. The crossword solution for this term is 'QUARK', which is a subatomic PARTICLE that is believed to be one of the fundamental building blocks of matter.