What does the term 'cipher' historically refer to?

The term 'cipher' historically refers to the concept of zero. In ancient mathematics, particularly in the context of Arabic numerals, the word "cipher" is derived from the Arabic word "sifr," which means zero or empty. The introduction of zero as a number was revolutionary, allowing for the development of place-value systems and arithmetic operations that include subtraction and more complex calculations. This concept transformed mathematical thinking and operations, enabling a clearer representation of numbers and quantities in various computations.

Additionally, while the other choices relate to mathematical concepts or operations, they do not align with the historical meaning of the term 'cipher.' For instance, a type of calculation or a mathematical operation pertains to the processes performed using numbers but does not specifically denote the concept of zero. Similarly, a symbol of equality refers to a different aspect of mathematics entirely, namely the symbol used to express equivalence between two quantities. Understanding 'cipher' specifically as the concept of zero highlights its foundational role in the evolution of mathematics.