Input size refers to the amount of data or information that is fed into a system, such as a computer program or algorithm. It can be measured in various units, such as bytes, kilobytes, or even the number of items in a dataset. The input size can significantly affect the performance and efficiency of the system, as larger inputs may require more processing power and time to analyze. In the context of computer science, understanding input size is crucial for optimizing algorithms. For example, an algorithm's complexity is often expressed in terms of its input size, which helps predict how it will perform as the size of the input grows. This is commonly represented using Big O notation, which provides a way to compare the efficiency of different algorithms based on their input size.