Pay down
Definition 1 of 2
Pro Tip 1/3Financial Focus
Used when discussing money management, especially paying off debt. 
She aims to pay down her credit card balances first.
Pro Tip 2/3Long-Term Process
Suggests an ongoing effort, not a single payment. 
I plan to pay down my car loan over the next five years.
Pro Tip 3/3Budget Planning
Often implies intentional budgeting to reduce debt steadily. 
They're budgeting extra each month to pay down their mortgage.