I was reading The Lathe Worlds and came across a term wich im not familiar, Tech-witch. What in the name of the omnissiah is a tech-witch?

That would an unsanctioned user of techmaturgy, the divine art of manipulating and enhancing the Machine Spirits through ritual prayer and canticles of the Omnissiah. But where the glorious followers of the Machine God give praise to the Spirits in this fashion, forging with it a wondrous communion, the Tech-Witch uses vile techno-sorcery to pervert Machine Spirits to do her bidding! With blasphemous rites of intrusion and curses of blindness she bends hapless technology to her will... a most detestable being, one any true Adept will hunt down and destroy for her treacherous ways!

tldr; They're 40k hackers

Vaguely, think of the difference between a "Priest" and a "Witch" in the religious sense. A Priest sees the divine as wondrous, something to serve, and possibly flow through themselves, to produce miracles the divine intends, while a Witch uses proscribed knowledge to pervert this energy, and either force it to their own will, or serve on behalf of their dark benefactor, who needs them to manipulate the divine, on their behalf. Please let's skip the real-world views of the above; I'm not trying to describe, nor judge anyone's real life beliefs, but in a literal sense, I feel I hit what many people would understand.

In this case, "tech" = divine. The AdMech uses tech to perform miracles, but often serves the tech, and accepts that it doled out the miracle, while the witch understands what the tech can do for them, and forces the right options, to perform the miracle, on demand, regardless of what the tech wants. Sort of how Jedi feel the Force works through them, while the Sith feel they are masters of the Force, and it serves them, because they are worthy, chosen by it. Hmm, I babbled. Oh well, here's hoping there is something to take from it.

Agreed. If you subscribe to the theories that A.) only a limited few people know how to maintain the higher-level tech aboard, say, a starship, and B.) that "typical" representatives of said organization will turn on you, under certain circumstances, if they believe you have turned on their beliefs, having some people who might 1.) be as "open minded" as you are, 2.) be knowledgeable of such technologies as previously mentioned, and on, can prove gratuitously beneficial. As I'm sure I've said elsewhere, it amazes me the number of people in the fluff who can both go completely off the rails the Emperor's little piece of paper to them provides, and do so with a mighty voidship that still works, even after numerous void battles with people who have, if you will, a differing of opinion. And maybe not quite so crazy to have as a Warp Guide, or whatever weird sorcerers kill one or more of your crew, to guide the ship, like a Navigator.

Wouldn't a tech-witch essentially be just another type of a heretek? I imagine that as far as anyone outside of the AdMech is concerned, the meaning of the words may be different. Actually, considering that many Imperials are much more familiar with the term "witch", I can see them calling any rogue technologist a tech-witch.

One beliefs Saint is another's Fiend.

Remember that many "more opened minded" or outright heretechs are still legitimate members of AdMech