Even Wikipedia can clear up the question. I would use my actual sources instead, but most people won't even read the wiki article completely.
If you leave aside all the angst, a few striking facts remain which put a very different light on the general word "lolita".
1- It's not about being "less than 18, boobs or not". Since the very concept of a "lolita" is based on the character depicted in that novel, expect a rather immature body and an age way below that. A lolita is real deep blue smoking dangerous jailbait.
2- A lolita is not just cute and attractive, she also has a psychological profile, which is of a manipulative temptress. Also, she might not always realize the extent and the possible consequences of her newly found power over that older perv she got there.
3- So there is a lolita there has to be a Humbert. And that's where, in my humble opinion, the whole "I tell you that loli is what a group of people say it is, because it's like that in manga" becomes a sterile blabber. Seduction, charm, sex-appeal, call these how you like, are purely subjective. Lolita could have been a 12 years old with precocious big jugs (some hormonal disorders can do that) or as flat as a plank. It's totally irrelevant. Some older man finds her sexy and she uses that in return.
In Japan, the whole rage for loli manga girls started because of their lousy censorship system. Depicting pubes was a no-no. So there, let's take loli characters, there will be no pubes. Oh and we need a name for that, so let's see, wasn't there that psychological term, lolita complex? Oh yeah, it sounds cool, let's use it. It's too long though, boss. Ok surel Let's call it lolicon and put a copyright on it since we're at it.
A lolita (and by extension a "loli", as it's plain the same thing) can't be just defined in the reductive japanese way and when it's got totally developped boobs it's not a loli.
Now, can we move back to just enjoying the materials?