Daniel Dennett has advanced the opinion that the evolutionary purpose of the cuteness response in humans is to make us respond positively to babies. This does seem plausible. Babies are pretty cute, after all. It's a tempting explanation.
Here is one of the cutest baby pictures I found on a Google search.
And this is a bunny.
Correct me if I'm wrong, but the bunny is about 75,119 times cuter than the baby.
Now, bunnies are not evolutionarily important for humans to like and want to nurture. In fact, bunnies are edible. By rights, my evolutionary response to the bunny should be "mmm, needs a sprig of rosemary and thirty minutes on a spit". But instead, that bunny - and not the baby or any other baby I've seen - strikes the epicenter of my cuteness response, and being more baby-like along any dimension would not improve the bunny. It would not look better bald. It would not be improved with little round humanlike ears. It would not be more precious with thumbs, easier to love if it had no tail, more adorable if it were enlarged to weigh about seven pounds.
If "awwww" is a response designed to make me love human babies and everything else that makes me go "awwww" is a mere side effect of that engineered reaction, it is drastically misaimed. Other responses for which we have similar evolutionary psychology explanations don't seem badly targeted in this way. If they miss their supposed objects at all, at least it's not in most people. (Furries, for instance, exist, but they're not a common variation on human sexual interest - the most generally applicable superstimuli for sexiness look like at-least-superficially healthy, mature humans with prominent human sexual characteristics.) We've invested enough energy into transforming our food landscape that we can happily eat virtual poison, but that's a departure from the ancestral environment - bunnies? All natural, every whisker.1
It is embarrassingly easy to come up with evolutionary psychology stories to explain little segments of data and have it sound good to a surface understanding of how evolution works. Why are babies cute? They have to be, so we'll take care of them. And then someone with a slightly better cause and effect understanding turns it right-side-up, as Dennett has, and then it sounds really clever. You can have this entire conversation without mentioning bunnies (or kittens or jerboas or any other adorable thing). But by excluding those items from a discussion that is, ostensibly, about cuteness, you do not have a hypothesis that actually fits all of the data - only the data that seems relevant to the answer that presents itself immediately.
Evo-psych explanations are tempting even when they're cheaply wrong, because the knowledge you need to construct ones that sound good to the educated is itself not cheap at all. You have to know lots of stuff about what "motivates" evolutionary changes, reject group selection, understand that the brain is just an organ, dispel the illusion of little XML tags attached to objects in the world calling them "cute" or "pretty" or anything else - but you also have to account for a decent proportion of the facts to not be steering completely left of reality.
Humans are frickin' complicated beasties. It's a hard, hard job to model us in a way that says anything useful without contradicting information we have about ourselves. But that's no excuse for abandoning the task. What causes the cuteness response? Why is that bunny so outrageously adorable? Why are babies, well, pretty cute? I don't know - but I'm pretty sure it's not the cheap reason, because evolution doesn't want me to nurture bunnies. Inasmuch as it wants me to react to bunnies, it wants me to eat them, or at least be motivated to keep them away from my salad fixings.
1It is possible that the bunny depicted is a domestic specimen, but it doesn't look like it to me. In any event, I chose it for being a really great example; there are many decidedly wild animals that are also cuter than cute human babies.
tl;dr: Cuteness is the word that we use when we want something to experience a feeling of safety or otherwise be more confident than we think they would feel without special effort to make them feel that way.
Thanks for expanding. I want to throw out a warning that we're treading dangerously close to the foul line, but I think we're still in-bounds.
I understand the general point that words can have different meanings, and I'm open to the possibility that I'm falling victim to the typical mind fallacy. I don't have any alternate meanings suggested yet, so I'm going to try to preemptively defend my definition below.
I want to test this hypothesis with a visualization experiment. I don't expect it will take longer than about 2 minutes to do all of the visualizations. This is the scene I want you to imagine: the person, animal, or object is standing or sitting, whichever can be expected of it. If it's a person, he or she has a blank, unsmiling, neutral, unaggressive facial expression. If it's an animal, its face is similarly at rest. It's facing either Data or Spock (take your pick). Imagine Data or Spock saying the sentence out loud to the person, animal, or object.
52" plasma television set - It's flipping through many channels, previewing each one for about a second; someone is channel-surfing. "You will be replaced by better, cheaper technology in less than a year."
Baby - "You would test very low on an IQ test. You will continue to be a net resource drain for several years."
Sexiest person alive - Doesn't matter who or what gender - this person is desired greatly, and desired primarily for their ability to satisfy you, personally, sexually. Take a minute and picture this person facing Data or Spock. "Your opinion isn't respected in virtually any matter; people agree with it out of hope they'll be able to sleep with you."
Bunny - "In a year's time, you will be harvested and your muscles will be cooked in a soup."
Cute boy or girl - Crucially, "cute" describes a particular type of attractive person. Imagine a person you would describe as cute, but not a person who is attractive who could not be described as cute. For me (and some others), "attractive but not cute" is a category that includes "hot," for example. If the word "cute" is a synoynm for "attractive" with perfect overlap, skip this question and note it below. If you imagined a girl: "You are valued for your womb and your abilities as a nanny. Men will want you for a wife but will consistently lust after other women for their sexual satisfaction." If you imagined a boy: "Women will tolerate your lovemaking, but you will be valued for your patience and because your timidity makes women around you feel outgoing, bold, and charismatic."
Hyena - "You will never have the opportunity to reproduce."
Tiny shampoo bottle - Imagine a small carnation-pink shampoo bottle, perhaps 2 inches tall. It has a white, spherical cap. The spherical cap has a very small, intricate, carnation-pink ribbon affixed atop it, as if it were a Christmas gift. "Throw this bottle away; its small volume makes it effectively worthless as a shampoo container."
An old man - Imagine an old man who could be perceived as "cute." Perhaps an old man, short, 90 years old, who walks very slowly, bringing his elderly wife, of roughly the same build, a plate with a sandwich on it, and he's torn the crust off the sandwich because he knows his wife doesn't like it. His hair is combed impeccably over his bald spot. His pants aren't long enough for his legs; they're "highwaters." After he sits next to her, he pats her knee. Now imagine this old man facing Data or Spock. "Your wife is still hiding love letters in her closet from her boyfriend before you, who left her. She still reads them and has never been as satisfied since marrying you."
A creepy old man - Leering, sexually active. "You give women the creeps, so you won't have sex again between now and when you die."
Did some of these statements seem meaner than others? Did any of these make you want to say to Data or Spock, "Don't say that!" or "You're going to hurt his/its feelings"? If so, which?
My hypothesis is that the following visualizations will incite, in the typical person, either slight anger at Data or Spock, an instinct to reassure the object at which the unpretty truth is directed, or in some other way some protective behavior, such as an urge to refute the hypothesis especially emphatically for that particular visualization, more than the others: baby, bunny, cute boy/girl, tiny shampoo bottle, old man. My hypothesis is that the following incited either zero emotional response or a non-negative emotional response: TV, sexy person, hyena, creepy old man.
Interesting. My empathy seems to be working in a weird way.