Question

What are base anhydrides?

Answer 1

They are usually oxides of metals, that can react with water to form bases, or behave toward acids as if they had.

Explanation:

Best known example is calcium oxide `CaO` which will react with water to form calcium hydroxide `Ca(OH)_2`.
Other metal oxides will not react directly with water, but they will react with an acid as if they were a base:
`ZnO+2H^+ ->Zn^(2+)+H_2O`
(compare to: `Zn(OH)_2+2H^+ ->Zn^(2+)+2H_2O`)
In this case you can 'deduce' the imaginary in-between step from oxide to base.