"In the Stealth Cruiser, get to sector 8 without jumping to a beacon with an environmental danger."
What's the game definition of 'environmental danger'?
Any advise/tip on how to achieve it?
As per the FTL Wiki:
Tactical Approach: You must not jump into an environmental hazard until sector eight which includes asteroid fields, ion storms, and a nearby sun. Usually one of the easiest because you will typically avoid these.
Notably this does not include locations where an enemy weapons platform (eg planetary defenses) are firing at you. Also, if an environmental hazard is overtaken by the Rebels, it stops being an environmental hazard, and you can cross it safely without failing the achievement.
The good news is that anything that is counted as a hazard will appear as such on the star map (provided you still have your Long-Range Scanners) so should be simple enough to avoid in most cases.
The bad news is that you may come across a situation where it is not possible to continue without hitting an environmental hazard - check out the below screenshot from Reddit as an example of this. When this happens, your only chance is to wait for a Rebel takeover of one of those stars, or to start again.