The term PART OF AN EINSTEINIAN EQUATION can be solved by the crossword solution MASS, which is a four-letter word. MASS is a fundamental property of matter and is one of the most important concepts in physics. It is a measure of the amount of matter in an object and is related to the energy of an object through Albert Einstein's famous equation, E=mc2. MASS is an essential part of this equation and is used to calculate the energy of an object.