A unit (UoM, Unit of Measure, Unit of Measurement) is a quantity value defined by convention.
Example: For the quantity mass m, one unit is defined with the English name kilogram, and the symbol kg. By convention, the quantity value 1 kg is defined to be the quantity value of the mass of a defined amount of a platin-iridium kept under well-defined conditions. See https://physics.nist.gov/cuu/Units/kilogram2.html
Other units for the quantity mass are the pound, symbol lb, the long ton, symbol lto, for the quantity volume the unit barrel, symbol bbl, or the u.s. gallon, symbol ugl.
Measurement standards then define conversion factors between units for the same quantity.