Variable currency
From ACT Wiki
The variable currency in a foreign exchange quotation is the currency which there is a variable number of.
For example in the quotation GBP/USD 1.4600 or 1 GBP = 1.4600 USD, the variable currency is USD; meaning the variable number 1.4600 US dollars is exchanged for one British pound.
Also known as the Terms currency or the Underlying currency.