surtax
An extra tax charged on top of an existing one.
An additional levy imposed upon an existing tax base or liability.
What that means in practice
It is layered onto a tax you already owe rather than being charged separately. Some states use one to fund a specific thing, and some school districts levy one as a percentage of the state tax rather than of your income.
Surtaxes are computed on either the underlying base or the tax owed, and the distinction matters by an order of magnitude — a rate applied to liability is not comparable with the same rate applied to income. ⚠️ Iowa's school district surtax is the standard example of the latter form.
Questions people ask
Who decides this — the federal government or my state?
Each level sets its own. surtax federally and surtax in your state are separate rules. They just share a label. That is why the two returns rarely agree.
Is this one of those terms that needs three other terms to explain it?
Not really. surtax is fairly self-contained. You can understand it without first learning a chain of other definitions. That makes it a good place to start if the rest of the vocabulary is new.
Is this the standard name for it?
It is. surtax is the name this rule generally goes by. Searching it will find what you expect. That is not true of every tax term — some carry three names, depending on who is writing.
At which level of government is this determined?
Income tax concepts are defined independently at federal and state level. States may conform to the federal definition, conform selectively, or define their own; conformity is legislated and can be static (tied to a fixed date) or rolling. ⚠️ A federal change therefore propagates to some states automatically and to others only by subsequent state action.
Does this term resolve independently?
This term resolves largely independently of other defined terms, with limited cross-references. It is therefore usable as an entry point; more heavily cross-referenced provisions generally require it rather than the reverse.
Is this designation the settled one?
This designation is the settled term of art with limited alternative usage, so it maps reliably to statutory and agency references. Terms with multiple colloquial designations require more care when searching source material.