I put this in the comments but on second thought, eh, I'll put it in a reblog:

I would say the answer really depends on where in the world you are, because American Orthodox Judaism tends to define itself more by practice, whereas former British Commonwealth Orthodox Judaism tends to define itself more by belief and/or community association. I say this as someone who is Orthodox with friends and family from around the world, some "Orthodox", some not, with varying degrees of observance, belief, and community association.

A Commonwealth Jew might drive to shul or use electronics on Shabbat, eat "kosher-style" rather than strict halachic kosher, etc, but they go to an Orthodox shul and wouldn't even think about going to any other kind of shul, and if asked would say that they're Orthodox if they had to label it, whereas an American Orthodox Jew might see that same Jew and would never think of them as Orthodox and assume they were Conservative or Reform. And on the flip-side, an American Jew might, out of necessity, attend a non-Orthodox shul or no shul at all, but consider themself Orthodox if they abide by Orthodox halacha, because for American Orthodox Judaism, it's practice that's considered the defining feature. And in Israel the distinctions and dynamics are also very different, because Jews are minorities in Israel and have far greater agency. In Israel there's also the added dynamics of sociopolitical labels and questions (are you pro or against mandatory conscription? do you believe in Rav Kook's vision of religious zionism? do you believe that Israel should be a Jewish theocracy or a secular democracy? etc.) which both diversify and reduce the kind of labels Jews might categorize themselves and others into.

Even more importantly, "Orthodox" as a label is not something that Orthodox Jewry unanimously chose for ourselves. In fact, it began as an exonym, used by other Jews to refer to the kind of Jews and Jewish theologies and ideologies they were opposed to. Plenty of Orthodox Jews are uncomfortable referring to themselves as "Orthodox" and might only do so if they need to clarify aspects of their identity to an outsider. Orthodox Jews might just call themselves religious ("frum" for Yiddish-derived communities) or label themselves by which specific mesorah and/or nusach they're part of (Chassidic (Satmar, Belz, Chabad, Ger, etc.), Litvish, Yekke, Yerushalmi, Dutch, Italian, Moroccan, Persian, etc.) and think that "Orthodox" is a reductive and possibly even offensive label.

I disagree that there's a straight answer to anon's question, there really isn't, because "Orthodox Judaism" is in no ways a monolith. It is a debate, and it's an ongoing debate.

It's actually quite relevant to me, because I'm most aligned with the Open Orthodox movement, who many in the Orthodox world decry as "not really Orthodox". There is an ongoing debate about where to draw the line, about who is considered Orthodox or not, and this has very serious implications in the halachic and political realms. Questions about marriage and conversion and immigration and conscription in Israel are very connected to anon's question. The question shouldn't be dismissed as something that's not a debate, because it is a debate, and it's a debate that is being hashed out in the Knesset and in synagogues, Batei Din, schools, and households around the world.

It's unfortunately not a question that can be easily answered by a couple of short paragraphs on a Tumblr post.

Yeah, Open Orthodox is absolutely defined by practice. I’ve seen other Orthodox Jews define Orthodoxy as those who practice kashrut, niddah, and Shabbos. Which is a definition based on practice, though it’s one I disagree with, because

1. With kashrut and Shabbos there’s a lot of different levels you can keep and you get into the argument about if someone who keeps kosher style is Orthodox or if someone who doesn’t keep chalav yisroel is disqualified
2. niddah is defined by heterosexuality, so this immediately disqualifies queer people who go to Open Orthodox shuls (such as myself)
3. Quite frankly, niddah is a private mitzvah and nobody but your spouse and mikveh attendant should know if you keep it, so I don’t think it’s a good way to tell if someone is Orthodox

But I also disagree with the idea that it’s the belief that halacha is unchanging because like… halacha verifiably changes. We used to have polygamy! We used to not do shomer negia! If we actually believed halacha was unchanging then we would at least not acknowledge any rulings after the Baal Shem Tov (at least if you’re Hassidic), and honestly we really shouldn’t acknowledge anything after the Talmud was closed, which discounts Rashi and the Shulchan Aruch.

Also like… any community of multiple people is going to have variation. I grew up in a Conservative shul and there were people who didn’t keep kosher at all and people who didn’t drive on Shabbos. It’s a similar phenomenon to what Magnetomagnificent notes about Commonwealth Orthodoxy. There’s a guy I know at my mom’s shul who is more machmir about keeping Pesach than I am but hates even Open Orthodox shuls because he hates going to shuls with mechitzas. Any time you are trying to categorize something where you have more traits than you have categories, you’re going to run into gray areas.

The reason I’m in the process of shifting from Conservative Judaism to Open Orthodox, for me personally, is purely about practice. I wanted to become more observant and Conservative Judaism was not accommodating to that and seems to be shifting away from observant practice of the kind I grew up with. For me personally it’s very much not about belief and I think focusing on that can cause Conservative Judaism to get lost in the conversation, because in the Conservative shul I grew up in the approach to belief and halacha wasn’t that dissimilar to that in Open Orthodoxy.