The "second digit" refers to the second number in a sequence of digits, typically in a multi-digit number. For example, in the number 345, the second digit is 4. This digit plays a crucial role in determining the value of the number, especially in place value systems where each digit's position affects its overall contribution to the number's total. In mathematics, understanding the second digit can help with various operations, such as rounding or estimating values. For instance, if you want to round 345 to the nearest ten, you would look at the second digit, 4, to decide whether to round up or down.