So what is NSFW or Adult?
Not safe for work is very vague. In many workplaces no reddit image is safe. In many others, including reddit itself, adult material can be part of the work. So what is adult? Everyone has their own opinion but we need a clear rule. So we use the same rule used by most social media sites, including Instagram and Facebook: female nipples or genitalia mean adult material. Images without them are not tagged as NSFW/Adult. Using the same rule Instagram uses has the advantage of making almost everything here non-adult (the exceptions are posts that escaped Instagram's controls).
Some people think that many images that aren't tagged as adult here should be. Some think that small bikinis should be NSFW. But how small the bikini could be? How to measure it? And bikinis are usually used in public with children around, so how could they be "adult material"? For other people lingerie should be NSFW because they aren't used in public. But in many cases lingerie is less reveling that some bikinis. In other cases you can't tell from a picture what is a bikini and what is lingerie. For other people, nude women should be tagged as NSFW. But in many cases even though they are nude they don't show anything. A nude woman showing only her head and shoulders isn't adult material. So the fact she is nude can't be the only criteria.
We know some people think this rule is misogynistic because of the "female nipples" and even make protests on Instagram about this. But this is the society we have.
Some people say they can't define it but they know when they see it. It is an old discussion, but can't be done because you can't make everyone follow some person's opinion and you can't ask that person every time before making a decision.
We need a clear rule that everyone can understand. So we decided to use the same rule used in Instagram, the source of the most of the material here. It is also used by most social media sites. So it's the closest we have to a widely accepted rule.
tl;dr: NSFW = Female Nipples | Genitalia
By the way I can't write NSFW on the title or reddit would make this post itself NSFW and I couldn't change it so it would be invisible for some people.