An ADDITION is a mathematical operation that involves adding two or more numbers together to get a sum. The crossword solution for the term AN ADDITION is ODE, which stands for Ordinary Differential Equation. AN Ordinary Differential Equation is a type of equation that involves derivatives of a function with respect to one or more of its variables. It is used to describe the behavior of a system over time and can be used to solve problems in physics, engineering, economics, and other fields.