A unit is a standard measurement used to quantify physical quantities such as length, mass, or time. Common units include meters for length, kilograms for mass, and seconds for time. Units help ensure that measurements are consistent and understandable across different contexts and applications. Units can be part of larger systems, such as the International System of Units (SI), which provides a comprehensive framework for scientific measurements. Each unit can be converted into others, allowing for flexibility in calculations. For example, 1 kilometer is equal to 1,000 meters, making it easier to compare distances.