The phrase "marginal rate" refers to the amount of change that occurs with each additional unit. It is often used in economics and taxation to describe how tax rates or costs increase or decrease with each additional level of income or production.
Do I understand the dangers of high marginal rates of taxation? (telegraph.co.uk)
I would pick emergency funds because interest income is taxed at marginal rates. (canadiancapitalist.com)
These rules assess tax at the top marginal rate on taxable dividends from a private corporation received by any child under the age of 18. (taxplanningguide.ca)