I have decided to undertake the quest of expanding upon the entries found in the Morgan and Silva University Lexicon, as per Palizche's comment on the question. There are three results in this dictionary for "income tax":
- tributum generale redituum- "a common tax of revenue" originally from Helfer
- tributum manupretii- "a tax of wages" originally from Helfer as well
- vectigal pro reditu- "a tax for revenue" originally from Levine
Now, I'm not here to debate the quality of the sources, as I do not feel qualified to comment on this. It is important to note however, that the MSUL is a compilation of definitions from various, more modern sources for modern terms. These definitions are from the second part of the lexicon, called the Adumbratio, which means they aren't necessary classical idioms, simply put, but they are still worth looking at. I will instead analyze the translations for how close they are to the English definition.
So, right off the bat, I prefer the second translation. Both the first and third translation use a word that means "revenue." It can also mean "income," but take a look at the literal translation of reditum: "that which must go back/again." This may seem odd, but to me it suggests a closer tie to revenue, as revenue is money given back to the seller in exchange for a product or service. Income, however, is different, in that it usually refers to money given in exchange for labor, however it can have a more loose definition. Compare these English definitions for the two terms (revenue and income):
Revenue- income, especially when of a company or organization and of a substantial nature
Income- money received, especially on a regular basis, for work or through investments
These definitions again show that revenue is more applicable to companies and the money they make for selling products or services, and income is more applicable to money received in exchange for work. Manupretium means wages, hire, or reward. An income tax is defined in English thusly:
tax levied by a government directly on income, especially an annual tax on personal income
Knowing the definition of income, this is a tax on wages. This is why I believe that the second translation is the best. Finally, to further put the nail in the coffin for the other two, "general" implies that the tax applies to everyone evenly, which, at least in the United States and most countries, not everyone pays an income tax at all or at the same rate as others. I also personally dislike the use of pro in the third definition as it somewhat implies the tax is for generating revenue in one interpretation. This is what taxes are for, but in that context it could be any type of tax really.
Again, the word reditum can mean tax, but I believe it closer means revenue. A tax on revenue could be a sales tax , which itself has its own MSUL definitions, or a corporate tax, which I have given my own translation for because there is not one in the MSUL.
- Sales tax = venalicium, tributum consumptionis, vectigal rerum venalium
- Corporate tax = tributum societatium
In summary, an "income tax" can be translated as tributum manupretii.