I find it unlikely that discussion on this site will be more informative to you than research on your own, beginning with searching the web for related terms.

Prior discussion. Also, the site search function (upper right corner of the screen) is your friend.

I also notice I am confused about objectification. I find it easy to point to different things and say this is objectification, and this isn't. But when I try to infer the distinction, I can't specify it accurately enough to make me feel like I understand objectification. This is a problem for me because sometimes people make me feel objectified, and I want to tell them what they are doing and why it hurts, but I don't think I can communicate that well at my current level of understanding.

Anyway, it is easy to point out some first order approximations to the concept, like that it has to do with acting in a way that diaregards one's agency, but I think it is a subtler concept than just that.

Why not ask which behaviors and attitudes inflict harm, and then seek a definition of objectification which makes it easier to talk about in that way? I've found this a useful mindset to apply to problems where the status quo reaction is to apply emotional words to emotional problems. It sounds like this is the kind of problem unwrapping you're looking for here, because like me, you worry about the real harm of real people, but are confused about the way that others are talking about it.

A helpful impart of the rationality community to me has been to give myself consent to think about problems in my usual mode, which is reducing them to the non-semantic realities that I actually experience. If a distribution of possible actions has a distribution of necessary outcomes, we have described the system, and then terminology is a useful tool, rather than a bludgeon to beat your social opponents with.

My suspicion is that "objectification" in the minds of social pioneers roughly maps to types of behavior that encourage false beliefs in the (perhaps unwitting) perpetrator. The false belief would be an archetype about a category (like a sex or a gender or our species) that makes it difficult to distinguish between the reality of the presented sexuality and other false, learned tropes that were previously taught (outside of real experience) about that category.

It becomes tricky when archetypes become stereotypes which then become social norms. This can make beliefs about a category actually true. The sentence "men like X" if expanded to "many men that I come into contact with prefer X and it is beneficial to our relationship to act as if this were so" can be a true belief, even if liking X is not a universal trait of men (i.e. it's a social norm ingrained into men by high-minded city-slickers, or insert-your-favorite-group-meme-here). What's important, then, is the real future relationships you might have with actual members of that category, not the idealized version of the category itself. And thusly important is the effect that an experience has on your actions toward those members.

What to you is appreciation of the nude form? Gazing with scientific interest at the biology of human anatomy when it happens to not be wearing cotton? Appreciation of your own sexuality when your eyes see a sexual image? Pornography use? Marble statues in museums?

What are the real outcomes of choosing these behaviors versus their alternatives? If chosen, what are the real outcomes of some mindsets and their alternatives?

Ozy Frantz had some good stuff on this but it's gone.

I'd say a lot of it seems to be about a lack of reciprocality.

Is it objectification if you disapprove of it, and appreciation of the nude form otherwise?

Where do we draw the line between Appreciation of the nude form/sexuality and Sexual objectification?

Easy. If your "appreciation of the nude form/sexuality" changes the way you interact with the person in a non-sexual context, you are engaging in objectification.

Depublishing.

In the future, if you're not sure about the suitableness of a topic on this site, I recommend asking before posting about it.
The Open Thread threads are a good place to do that.

WRT the topic itself, I encourage thinking less about drawing abstract lines between ill-defined categories, and thinking more about the likely consequences of actions.

You say "sexual objectification" like it's a bad thing. Like for anything else with confusing connotations, the standard reply is "taboo objectification".

The clearest explanation I've seen of this issue is this Pixel Poppers post.

I see it this way. It is "objectification" when it's used to attract attention. It's "for the purpose of appreciation" when it's used to enrich emotional reaction (usually of the aesthetic evaluation, but sometimes of the moral evaluation). So it is hard to say just by the content, but if the content is both erotic and boring it's objectification.