Meaning of "Base" in Walsh Hadamard Tranform in Image Processing?
I believe that here bases stands for "basis vectors". From the cited paper by  Y. Hel-Or and H. Hel-Or. Real time pattern matching using projection kernels, 2003, ICCV,  you can see the set of $8 \times 8$ vectors from  a separable Walsh-Hadamard basis:

[20] Y. Hel-Or and H. Hel-Or. Real time pattern matching using
projection kernels. In ICCV, pages 1430–1445, 2003.

It consists in a tensor product of two $8$ 1D Walsh-Hadamard transforms (one horizontal, one vertical).



The sixteen 2D vectors  in the left-top $4\times 4$ square are the one with the lowest sequencies (sequency is the  frequency analog for Walsh-Paley-Hadamard transforms), in terms of "maximum sequency":