Final answer:
The base SI unit for mass is the kilogram (kg), which, from May 2019, has been defined by fixed values of fundamental constants, moving away from a physical object standard.
Step-by-step explanation:
The standard SI unit for mass is the kilogram (kg). This unit was previously linked to a physical object; a platinum-iridium alloy cylinder, but since May 2019, it has been redefined based on fixed numerical values of the Planck constant among other constants. This ensures the definition of the kilogram remains constant over time, as opposed to the slight changes in mass that occurred with the physical object due to contaminants adhering to its surface. The gram is a subunit of the kilogram, equivalent to 1/1000th of a kilogram. Pounds and tons are not SI units, with one kilogram equaling about 2.2 pounds.