It is mentioned in the GloVe: Global Vectors for Word Representation. It says:
where $w_i$, $w_j$ and $\tilde{w}_k$ are all word vectors and $F$ is just an unknown function. The author then assumes $F$ as $exp$. What is homomorphism exactly? Why equation (4) can make $F((w_i - w_j)^T\tilde{w}_k)$ homomorphism?
copying verbatim from Wikipedia