In topology, a base (topology) for a topology is a set of open sets such that every element of the topology is a union of the base sets. See also subbase.
In Marxism describes the material equipment and material relations of human society, as distinct from the 'superstructural' forms of society - laws, customs, ideas, beliefs, etc.
As modified by the word tax, it refers to how much income and assets one has, earns, spends, inherits, etcetera as used in the formula to decide owed tax (example: owed_tax = (base times tax_percent) minus deductible).