second mortgage
A mortgage taken out on a property that already has an existing mortgage; the second mortgage is subordinate to the first.
When you set up a second mortgage, its repayment priority is lower than that of the first mortgage holder.
抵当 is a general mortgage or pledge, while 二重抵当 specifically refers to a second mortgage on the same property.
根抵当 is a revolving mortgage that secures a fluctuating debt up to a maximum amount, unlike a standard second mortgage.
Compound of 二重 (double, two-fold) and 抵当 (mortgage, pledge). The term is a direct calque from the English legal concept 'second mortgage'.