I don't think there's one single definition of "professor" that is true around the world.
The idea behind adjuncts was originally that you might hire someone on a temporary basis, either because of an unexpectedly large class or to offer a class on a special topic: a famous politician might offer a seminar about their time in office or an industrial researcher might offer something related to their work.
However, it’s become increasingly common to hire adjuncts to teach intro classes because they’re cheaper than professors (usually a flat fee per class and minimal benefits) and don’t involve much commitment (usually hired per semester), both of which have obvious short-term benefits to the administration.
It's possible to treat teaching faculty well. And even necessary in some in-demand fields where the private sector options remove the option of treating non-traditional faculty like crap.
(I guess not any instructors because there are TAs and the like, but people leading lectures or similar)
In the US at least, there are thousands of (good) Bachelors, Masters, and sometimes even Ph.D.-granting colleges/universities that do not require an R1-level research agenda for the Full Professor rank.
And many students use "Professor" as a title for any instructor above the Highschool level, even for people who do not have the word "Professor" in their job title, such as ad juncts, full-time non-TT instructors, and sometimes even TAs.
This answer is US-specific
As I understand the US system for four year universities
TOP Professor with an endowed chair University or Full Professor
Associate Professor (They have tenure and time in position)
Assistant Professor (They don’t have tenure. This is their first job and they want tenure here.)
BOTTOM Visiting Assistant Professor (They got denied tenure at a higher ranked university than they currently teach at and want tenure at that university or another one of similar rank.)
Adjunct Professor Someone with a non-academic job, expertise or position who teaches a university course because they enjoy doing so. Also a deluded holder of a Ph.D. who thinks this is still a part of academia. It is not. That is why it is below the bottom. These people are highly qualified, poorly compensated casual labour, desperately clinging to hope of an academic career after this makes any sense.
Anyone who describes themself as an adjunct professor as if it’s sonetbing other than a hobby should be encouraged to get a job that isn’t shit. Teaching high school is vastly less intellectually rewarding but the holidays are better and so is the insurance and the pension.
I looked into this in my state. Getting your foot in the door as a teacher in a public school would require someone with a PhD in say, mathematics, physics or computer science, to go back to grad school for a masters in education. Not sure that's an option, and is also ridiculous considering I just spent six years in grad school. You could do TFA as a backdoor, in many states, but that is probably an even worse deal for you physically and emotionally than being an adjunct.
But you need to get your resume in front of the right people -- namely, folks in the central office (or maybe principals) who view your area as a strategic/difficult hiring area. Easy peasy in CS these days...
Also check out private schools.
My understanding is that private (k-12) schools pay dramatically less than (unionized) public schools. I've had family and friends who taught even at very expensive secular private schools, who then moved over to public school because the pay, pension and benefits were dramatically better.
I don't have hard numbers, but my impression from talking with family and friends is that you are much better off as an adjunct at a public college than as a k-12 teacher at even a prestigious private school or non-union charter school, though a union teaching job at a public k-12 school pays better and is much more stable than both.