Tax Base

The assessed value of a set of assets, investments or income streams that is subject to taxation, or the assessed value of a single asset that is subject to taxation. Anything that can be taxed has a tax base.

Investopedia Says:

The tax base may refer to that of an individual asset, such as the tax base of a house, or a pool of assets, such as the tax base of all houses in a city. For example, the property tax base of a house is its value. The property tax base of a city is the collective value of all taxable real estate in the city.

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Total amount of taxable property, assets, and income that can be taxed within a specific jurisdiction. A town's tax base is the assessed value of the homes and apartments (minus exempted property), income from businesses, and other sources of taxable activity. If a business moves out of the town, the tax base shrinks, shifting the tax burden onto remaining homeowners and businesses.

The collective value of property, income, or other taxable activity or assets subject to a tax. Tax revenues are computed as the tax base times the tax rate. For property taxation, the tax base is the total assessed value of all taxable property less exemptions.
Example: American Widgetworks plans to build a new plant in Centerville. The plant will add $15 million to the property tax base of the city.

The noun has one meaning:

Meaning #1: collective value of taxable assets