base rate

Short definition from Wiktionary

Noun

1. 1
   A benchmark interest rate, from which other interest rates are calculated; a policy rate.
2. 2
   A benchmark interest rate, from which other interest rates are calculated; a policy rate: Especially, the interest rate at which the Bank of England lends money to commercial banks.
3. 3
   The background rate of occurrence of any phenomenon, which informs any logical analysis of its rate of occurrence under any subset of circumstances; especially, the proportion of individuals in a population who have a certain characteristic (e.g., trait, experience, circumstance), irrespective of any treatment effects in a treatment group of a scientific study.