Used awkwardly -- and ONLY awkwardly -- after something you've said to/about the opposite sex (or same, if that's how you go about) could be mistakenly taken into a flirtatious or more-than-friendly context. Replaces "but not like that" in most cases.

Basically:
"I'm completely positive towards you at the moment, but not in a way that I want to shove my crotch into your face."

The safeguarding of Earth and other worlds from biological cross-contamination (i.e. billionaires with too much time on their hands).

Also known as “planetary protection.” Planetary protection / quarantine “reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.”

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.