velut

Velut is vel plus ut, "just as, like". The longer veluti means exactly the same thing and exists mostly to give poets an extra syllable, so treat the two as one word.

It overlaps sicut and tamquam closely, and the difference is register rather than sense: velut is the poets' particle and Virgil's standard way of opening a simile, while sicut is the plain prose and biblical choice. The syntax matches tamquam. A bare noun phrase keeps its own case, and velut si introduces an unreal comparison with the verb in the subjunctive. Nothing in Romance continues it - Spanish como and French comme both descend from quomodo instead.