Mabu-dachi (マブダチ) in older Japanese slang that stands for good friends, best friends or real friends. Mabu (マブ) is a peculiar way of saying true or real, dachi coming from tomodachi (友達, ともだち or トモダチ.) Thus it’s easy to take that and give it a slight change into Mabu-Love, which would then stand true love. Corrupting terms, words or even sentences is nothing new to Japanese, or other languages in general, to make new terms or giving them usages. Seeing how staff at âge are bunch of fanboys from the days mabu-dachi saw most popular use, from the 80’s and 90’s respectively, it’s easy to see this then popular slang being used. Mabu-dachi is used in various other products as well, like a 2001 movie using it as its title.
The title is reflected in the catchphrase Save in the name of true love, which can be seen in the franchise’s official logo to this day.
The final nail in this question, for those who are still not convinced, comes from Kimi ga Nozomu Muv-Luv.
The furigana for 真愛 is Muv-Luv, and these two stand for True and Love respectively. There’s no doubt about the meaning of Muv-Luv any further.
What does True Love in regards of Muv-Luv stands for? There are as many different arguments as there are people reading the visual novels, and so I can’t really say anything for sure, but I’ll list a few of them.
Perhaps most divisive argument that the title refers to Sumika. She is arguably Takeru’s true love. We can also argue that because mabu-dachi is slang for true friends, we can see connection between their friendship and their romantic love. Alternative is also all about finding that one thing that can break the loop and send Takeru home. We know that is his true love that can do that, but she needs to be saved, in a sense.
Argument can be made for Meiya as well, as she is one of the two characters whose route needs to be completed in order to unlock Unlimited. Her childhood promise with Takeru can also be seen as a pact made in true love. Such love would survive through times in romantic sense, after all.
The rest of the girls, whether or not they fit the bill is an open question, one which can be turned a bit around. What if the one true love here is Takeru? It could be seen so that as long as Sumika is present, Takeru is locked with her by his own volition and feelings towards her. With all the alternative worlds’ Takerus being one being in BETAverse, all the options are more or less Unlimited. Of course Extra has routes for all the female characters, but even then this take would hold water as we can assume each route representing one parallel world with different outcomes for whatever reason. Nevertheless, it would seem that Takeru is destined to end up with Sumika, but it’s always nice to see âge leaving such things absolutely and completely open for debate, which fans have made into certain level of warfare.
Nevertheless, Takeru being the true love would stand in Alternative too. While I’ve seen the subtitle standing for the alternative route the story takes to prevent The Day After events, which does, it can be said that it refers to the girls taking action in order to save Takeru. He being their true love, he is the one who needs to be saved at the very end. Even if it costs their lives.
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[8.1.2016 addition]
I was supposed to modify this post earlier, but you know my memory. Yoshimune Koki has confirmed the meaning of the title few times around, but perhaps most recent was during Kickstarter, when he mentioned that Muv-Luv stands for true love. In addition, in one of the Kouhou no Houkou, or Roar of Publicity he mentioned that Sumika is indeed the titular true love.
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I’m not interested in discussing who is the best girl or anything like that. That’s completely up one’s own opinion. What the visual novels say is another thing altogether, and how we interpret them is a whole ‘nother can of worms. Still, I can imagine we all can make our own mind to what the title refers to.
One thing I’d still want to put into the spotlight; how does the title hold up with the later instalments of the franchise?
In reality, Muv-Luv after the original visual novels is just the name of the franchise. While both of the two more important spin-offs of the original stories, Total Eclipse and the upcoming Schwarzesmarken, have romance as one of their major themes, it’s more about carrying the theme of Muv-Luv in general than being about one true love. Then again, in Total Eclipse we do see Yuuya ending up with Cryska, and it’s mutual by all means. The animation doesn’t go that far, so in that we see Yuuya wanting to bone Shiranui Second Phase 2.
The most interesting thing with this is that Schwarzesmarken has no Muv-Luv in it. It’s just Schwarzesmarken. It’s almost like they want to make a clear distinction with it, which isn’t all too uncommon, but always a bit surprising. I know from second hand information what Schwarzesmarken is all about, but seeing how I haven’t read the actual thing myself, I’m leaving this bit open until the visual novel is released sometime in the near future. Knowing âge, it’ll be pushed back at least two times and sees the daylight sometime in 2016 rather than this year. Nevertheless, I’m sure I’m going to have some fun discussing it with a friend who is pretty well versed with events and stuff that went on between the split Germans.
As a side note, Schwarzesmarken is Engrish. It should be something like Die Schwarz Marks, The Black Marks. Essentially, âge fucked this one up, like Valkylies, but this time gave it an in-universe explanation later on; it was a Russian who gave them the name, screwing it up. Eh, it works.
I wanted to make some kind of jokes about True Love and Muv-Luv, but there’s very little one can make jokes out of. Perhaps the one thing the two have in common is that outside the whole romance thing is that we don’t have one true love, unless we so choose and decide to grasp it. In love stories this is different. My own take is that Mabu-Love is, at its core, a story about love and loss between Takeru and Sumika. That’s just me, and whatever you hold true is just as valid, unless somebody at âge decides to make definitive statements about things.
Aaltomies 
well… i thought that Muv Luv meant something like Multiverse Love?
I understand the logic behind someone making that guess, but it’s just a guess in the end and not a correct one either.