SI Units
SI units are the internationally agreed system of measurement used in science, built on base units such as the meter, kilogram, and second.
Definition
SI units are the measurement units scientists everywhere agree to use so that everyone's numbers mean the same thing: meters for length, kilograms for mass, seconds for time. The International System (SI), the modern form of the metric system, defines seven base units (meter, kilogram, second, kelvin, ampere, mole, candela), and other units such as the newton and the joule are derived by combining them. SI is coherent, meaning every derived unit is a product of powers of the base units with no numerical factors, so equations can be written without conversion constants. Since 2019 all seven base units are defined by fixing the values of physical constants, making them reproducible in any laboratory rather than dependent on a physical artifact.
Example
In a science lab you measure the ramp in meters or centimeters, the cart's mass in grams, and time in seconds, not feet, ounces, and heartbeats. Density combines mass in kilograms and volume in cubic meters to give $\text{kg/m}^3$, and water's $1.00\ \text{g/cm}^3$ equals $1000\ \text{kg/m}^3$. Because $1\ \text{N} = 1\ \text{kg} \cdot \text{m/s}^2$ and $1\ \text{J} = 1\ \text{N} \cdot \text{m}$, a $0.50\ \text{kg}$ cart raised $0.40\ \text{m}$ gains $mgh = 0.50 \times 9.8 \times 0.40 = 1.96\ \text{J}$ with no conversion factor anywhere.
Key Insight
SI stands for the French name Système International, and it is why a scientist in Charlotte and one in Tokyo can compare results without converting anything. The kilogram, not the gram, is the base unit for mass, the only base unit whose name carries a prefix. Mixing unit systems inside one calculation is a classic source of silent error, and converting everything to SI first removes an entire category of mistakes.