The term 'crucial element' can be broken down into two crossword solutions: KEY (3 letters) and ESSENCE (7 letters). KEY is a noun that refers to something that is necessary in order to gain access or to solve a problem. It can also refer to a person who is important or influential. ESSENCE is a noun that refers to the most important or essential part of something, or the quality that makes something what it is. Together, these two words describe something that is absolutely necessary and fundamental.