Tracker Mortgage
A variable rate loan with an interest rate that's at a set amount above or below the Bank of England or some other base rate, set independently from the lender. It tracks (moves up or down with) that rate
There are often penalties, sometimes during any special deal period, and maybe even after the period too.
It can pay to go for a tracker if you can afford to pay more when interest rates go up, in exchange for benefiting when they go down.
It's not a good choice if your budget won't stretch to higher monthly payments.