Tax base的定义是：

Tax base is defined as the income or asset balance used to calculate a tax liability, and the tax liability formula is tax base multiplied by tax rate. The rate of tax imposed varies depending on the type of tax and the tax base total. Income tax, gift tax and estate tax are each calculated using a different tax rate schedule.

这一概念相当于汉语的“计税依据”：

计税依据又称税基，是计算应纳或应征税款所依据的标准，是根据税法所确定的，用以计算应纳税额的基数。应纳税款是依照计税依据乘以适用的税率计算得出的。没有计税依据，则无法计算应纳税款，就无法确定纳税义务的内容。

比如，个人所得税的计税依据，就是毛收入减去税前扣减和各种费用，也即taxable income。Tax base还有另一个意思：

The value of all assets that a government may tax. The tax base may increase for a number of reasons, particularly with the creation of wealth or when persons with high income move to an area. The tax base is particularly important to local governments because persons with large amounts of assets can move in and out with relative ease. The tax base is also the reason that government revenues tend to increase during economic growth and shrink during recessions.

这一定义相当于汉语的“课税基础”：

课税基础又称税基，指建立某种税或一种税制的经济基础或依据。例如，流转税的课税基础是流转额，所得税的课税基础为所得额，房产税的课税基础为房产等等。选择税基是税制建设上的一个重要问题。选择的课税基础宽，税源比较丰富。

所以tax base可以译为“税基”，或视情况分别译为“纳税依据”或“课税基础”。

容易与tax base混淆的是一个会计上的概念，叫作tax basis，也叫cost basis。比如：

Where an asset is purchases, tax basis generally includes cash paid plus liabilities assumed. For example, if Joe acquires a building for $10,000 cash and assumes a mortgage for $80,000 (which is his liability assumed), Joe’s basis in the building is $90,000.

这个概念中文叫作“计税基础”，是2006年发布的《企业新会计准则》中提出的概念。它分为资产的计税基础、负债的计税基础两类内容。

参考文献：李长栓《法律术语翻译二十讲》