1. Tax - gains.
In relation to tax on gains, usually on the disposal of assets, base value is the amount by reference to which any chargeable gain will be calculated.
As a starting point, any excess of disposal proceeds over the base value might be potentially chargeable to tax.
Sometimes known as basis value, depending on the tax jurisdiction.
2. Changes over time.
Any amount or value used as part of a calculation of change over time.
For example, to calculate the level of an index.