The term 'Number Under a Line' is a mathematical concept that refers to the process of subtracting one number from another. It is also known as subtraction and is an important part of basic arithmetic. Subtraction is used to find the difference between two numbers, or to determine how much one number is greater than another. The answer to the subtraction problem is the number that is under the line. For example, if you subtract 5 from 10, the answer would be 5, which is the number under the line. The crossword solution for this term is TOTAL, which is an acronym for the phrase 'Take One Away To Leave'.