Definition 3. Remember, the English you speak isn’t the exact same as English spoken over 2 centuries ago, in this context the obvious and predominant meaning at the time of the writing of the 2nd Amendment is that “well-regulated” didn’t mean “regulation” as you imagine it now, it was more along the lines of well-functioning/trained/maintained/whatever.
But the meaning isn’t even relevant because the “right to bear arms” isn’t bound by it:
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
From a linguistically unbiased standpoint, it’s clear that the first half, “A well regulated Militia, being necessary to the security of a free State,” is a reasoning for the directive, “the right of the people to keep and bear Arms, shall not be infringed.” The usage of commas has changed over time, which is where a lot of the confusion comes from nowadays, a more modern reconstruction would only use one comma.
The term for it would be “absolute clause” – it serves many purposes, and in this case it gives reasoning for a something, but doesn’t lock that something to the reasoning.
Politics has seeped deep into peoples’ view of the linguistics of the amendment, but it’s really simple, this is basic grammar. It doesn’t say nor imply “The right of a well-regulated Militia to keep and bear arms shall not be infringed”, they specifically wrote it as “the right of the people” for a reason.
Making it an argument of the 2nd Amendment only applying to militias is arguing in bad faith – it’s clear that the amendment was written for everyone to have the right to bear arms, regardless of militias (although motivated by the security of the state, which well-armed militias can supply).
The only argument is whether the 2nd Amendment is suitable for the modern day, whether we should repeal/overwrite it, or at the very least to what extent protecting the “right of the people to bear arms” can be applied – obviously prisoners/felons can’t bear arms, there are a lot of regulations on who can bear arms and which arms you can bear (not even “the militia” can just bear any arms they like). And of course other first world countries are faring much better without a “2nd Amendment”, and with much tighter gun control.