1) What do you consider to be stalking online?
Reposting their pictures online, monitoring activity, trying to be in the same public spaces as them "by coincidence," locating mutuals, family, work, etc. Stalking is when the activities become suspicious beyond any need for vetting someone for safety purposes, and the information they gather is instead used maliciously such as to intimidate or blackmail.
2) What do you consider to be stalking in person?
Much of the same actions online can be done in-person, the difference is in-person adds the additional threat of that person being close you and where you live. In addition to the above in person stalking would include following you, tracking your routines, spying and stake outs, etc. In person has additional risks in assault, battery, kidnapping, and murder.
3) Have you or someone you know ever dealt with a stalker? If yes, then how was the situation handled?
I've known a few folks that have had stalkers. I myself had some online stalking/blackmail incidents as well. I actually have a couple pics posted to someone's personal XVideos, benign pics of selfies, no nudes. It's for these reasons I have strict rules against sending any nudes, and I'm currently pursuing AI generated photos to further take myself offline and use likeness.
I think the solution in all those cases was to go dark while it blew over. Never give into the demands because either you're giving them more ammo and/or showing them that they have power over you that they don't have to get rid of. Don't trust promises to remove it either. It's on the internet forever more than likely, but the Internet is so vast, your pics and info will be buried eventually. In the meantime, stay safe. If you think your home has been compromised, consider staying at a friend's for a while, consider moving, etc. It will die down, but before it does, you need to keep yourself safe.
4) What do you do to prevent or lessen the chance of being stalked?
The actions to take to lessen stalking are the same as reducing your digital footprint. You digital footprint are the many traceable actions you leave behind online. Accounts, posts, activities all are used to create a unique dossier of your information that major information firms such as Google/Facebook sell online. The primary use of the digital footprint is to sell you targeted advertising, but if the information is publicly accessible or on the dark web following a breach of a lesser service, then malicious users can also get their hands on this info and eventually reverse engineer their way to finding your real identity. So what you do in reducing your digital footprint is you remove these publicly accessible information by removing accounts, controlling who can see your posts in the first place, and creating your own dossiers completely separated from your real identity. If you don't think your info is compromised right now, it is. You can look for yourself at
https://haveibeenpwned.com/
Burners. Accounts, names, phone numbers, emails that are separate from your personal identity. Tools will give you voip numbers while you can spin up as many gmails as you want. Essentially create a fake persona: burner numbers and emails, and fake names, birthdates, and locations. Don't give them anything with which to find you. In the US you can get a P.O. Box instead of registering your addressing as well, and there are tools to further remove your address from places if may be posted.
Consider things like freezing accounts from companies that track and sell your data such as TWN here. In the US, you can freeze your tracking info with Equifax, which is tax and work-related information that they send to TWN and sell to other companies, thus propagating your info into more places online when you Google search your name. If someone finds your info, it's outdated and you're still relatively covered from some blackmailer. Use less pics. I run my pics through some image searches to see what comes up. My two favorites are Google images and Yandex. I did this a couple months ago and proceeded to remove every one of my accounts that showed up. Haven't seen a pic of myself since. I do the same process to vet someone's pics when they send them to me but their "phone camera is broken" or some other bs. It's not a fool-proof system, but it's the best I've found. Also keep a tab on pics you think might be "compromised," that is you sent them out to someone on good faith that turned out to be a scam or collector. If it's the same pic, then there's a lot less to build an online dossier around your persona. I don't mind sending the same pic to someone over and over again cause they message me from new accounts, making sure that pic is not connected to anything else.
Even on video, people can take screenshots of you. I'd say dress modestly and keep it safe for work. Don't send nudes or play online until you're good and ready to trust each other. If you do play online, conceal your identity. Masks, hoods, and sunglasses are all good ways to do that. You could also go even farther with hair, makeup, and colored contacts to further distinguish yourself from your professional profile.
Last thing to consider is to accept the rise of AI and deep fakes. Anyone right now could make porn of you. As that idea catches on, it should throw into question the legitimacy of anything found online. For people already posted online, this can be a comforting thought as if anyone found your pictures, there'd be no way to tell if it was real or an AI deep fake somebody made with your likeness.