The term 'PRINCIPAL ELEMENT' can be broken down into two words: 'PRINCIPAL' and 'ELEMENT'. PRINCIPAL is an adjective meaning 'most important, chief, or primary'. An ELEMENT is a substance that cannot be broken down into simpler substances. Therefore, the term 'PRINCIPAL ELEMENT' can be defined as the most important or primary substance. The crossword solutions for this term are 'STAPLE' (6 letters) and 'BASE' (4 letters). STAPLE is a noun meaning 'a thing that is essential or indispensable'. BASE is a noun meaning 'the lowest or bottom part of something'. In conclusion, the term 'PRINCIPAL ELEMENT' can be defined as the most important or primary substance, with the crossword solutions being 'STAPLE' and 'BASE'.