as opposed to the Agente de Retención/the Withholding Agent paying over the tax.

Taxpayer meaning the taxable person.

Two words, same as the source, which is itself a jargonized abbreviation.
It means the person (taxpayer) from whom the tax is withheld.
Sujeto = sujeto pasivo (taxpayer)
Retenido = withheld (from the tax is withheld).

NB: All the other answers are correct.

--------------------------------------------------
Note added at 7 hrs (2016-05-03 07:15:20 GMT)
--------------------------------------------------

Retenido = withheld (from WHOM the tax is withheld).

I would suggest

--------------------------------------------------
Note added at 4 hrs (2016-05-03 04:18:38 GMT)
--------------------------------------------------

lamentablemente me toca pero al fin y al cabo hay que pagar

--------------------------------------------------
Note added at 14 hrs (2016-05-03 14:07:00 GMT)
--------------------------------------------------

A withholding tax, also called a retention tax, is a government requirement for the payer of an item of income to withhold or deduct tax from the payment, and pay that tax to the government. In most jurisdictions, withholding tax applies to employment income. Many jurisdictions also require withholding tax on payments of interest or dividends. In most jurisdictions, there are additional withholding tax obligations if the recipient of the income is resident in a different jurisdiction, and in those circumstances withholding tax sometimes applies to royalties, rent or even the sale of real estate. Governments use withholding tax as a means to combat tax evasion, and sometimes impose additional withholding tax requirements if the recipient has been delinquent in filing tax returns, or in industries where tax evasion is perceived to be common.

Typically the withholding tax is treated as a payment on account of the recipient's final tax liability. It may be refunded if it is determined, when a tax return is filed, that the recipient's tax liability to the government which received the withholding tax is less than the tax withheld, or additional tax may be due if it is determined that the recipient's tax liability is more than the withholding tax. In some cases the withholding tax is treated as discharging the recipient's tax liability, and no tax return or additional tax is required.

The amount of withholding tax on income payments other than employment income is usually a fixed percentage. In the case of employment income the amount of withholding tax is often based on an estimate of the employee's final tax liability, determined either by the employee or by the government.

--------------------------------------------------
Note added at 14 hrs (2016-05-03 14:08:24 GMT)
--------------------------------------------------

and to answer your question: party = taxpayer