Base unit
A base unit is the single reference unit a conversion passes through — metres for length, kilograms for mass — so any unit converts to any other without drift.
Also called SI base unit, reference unit
Converting directly between every pair of units would need hundreds of factors and invite error. Instead each unit is defined against one base unit, and conversions pass through it.
A mile becomes 1,609.344 metres; those metres become 1.609344 kilometres. Because both steps use exact definitions, the result matches the definition rather than compounding rounding.
The SI base units are the foundation: metre, kilogram, second, ampere, kelvin, mole and candela. Everything else — newtons, litres, joules — is derived from them, which is why one clean set of factors covers the whole family.
Common questions
Why convert through a base unit?
One exact factor per unit keeps conversions consistent. Converting miles to kilometres through metres avoids the rounding drift of chained estimates.
Which units are SI base units?
Metre, kilogram, second, ampere, kelvin, mole and candela. Every other SI unit is derived from these seven.