From IVoM:

base quantity
quantity in a conventionally chosen subset of a given system of quantities, where no
subset quantity can be expressed in terms of the others

NOTES
1 — The subset mentioned in the definition is termed the set of base quantities.
EXAMPLE
The set of base quantities in the International System of Quantities (ISQ) is given in 1.6.
2 — Base quantities are referred to as being mutually independent since a base quantity cannot be
expressed as a product of powers of the other base quantities.
3 — “Number of entities” can be regarded as a base quantity in any system of quantities.

Indicate here with 'base' attribute of Quantity.

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