What is the meaning of root -vad- in words such as invade and evasive?

The root "-vad-" in words like "invade" and "evasive" comes from the Latin verb vadere, meaning "to go" or "to walk".

Here's how it breaks down:

* invade: "in-" meaning "into" + "vadere" meaning "to go" = to go into (a place, often with force).

* evasive: "e-" meaning "out" + "vadere" meaning "to go" + "-ive" meaning "tending to" = tending to go out of the way, avoiding.

So, basically, "-vad-" signifies movement or direction, which can be applied to various situations like going into something, going out of the way, etc.