1. Let's clarify the question: you are asking about the origin or meaning of the equal sign in the expression "5.1". 2. The notation "5.1" represents a decimal number, specifically the number five and one tenth. 3. The equal sign (=) is not present in the expression "5.1" itself; instead, it is used in mathematics to state that two expressions represent the same quantity. 4. For example, if we write $5.1 = \frac{51}{10}$, the equal sign indicates that the decimal number 5.1 is equivalent to the fraction 51 over 10. 5. Therefore, the equal sign is a symbol indicating equality or equivalence between two mathematical expressions. 6. It originates from the concept of balance or sameness and is used to show that what is on the left side has the same value as what is on the right side. Final answer: The equal sign (=) is a mathematical symbol used to indicate that two expressions represent the same value or quantity.