Something strange unfolds online each spring. As the warmer months approach, many men seem compelled to post about the allure of a woman in a sundress. The simple wardrobe staple has long been a point of inexplicable obsession, but this year, people are asking questions.

Why do some men get so excited to see sundresses? Wait — do men even know what a sundress is? Does anyone know what a sundress is? As social media flooded with responses, it became clear that no one could quite agree on what made a sundress a sundress (as opposed to a slip dress, a day dress, a shift dress, a shirtdress, a caftan, a tube dress or a nap dress).

So we want to unravel this thread a bit, and ask you, the reader, to answer the question at hand: What is a sundress?

Many people say sundresses are bright and floral, maybe blue or yellow. White is widely accepted. Pastels are classic. Black is divisive. No one really talks about gray.

On the resale platform Depop, a seller named Bianca Steele listed a “Boho Black Sundress 100% Viscose sundress made in India.” The inky maxi was “most definitely” a sundress, Ms. Steele wrote over the in-app messenger, adding that she had personally enjoyed black sundresses for over four decades. She currently owns at least 10.

But Jeannie Stith, the chief executive of Color Guru, a seasonal color analysis company, said she can’t condone a black sundress. “In general, black has been sold to us as a universal color,” she said. “It’s actually not.”

Ms. Stith said that universally flattering shades had a mix of warm and cool tones. For sundresses, that includes peony, periwinkle, teal and sage.

While out in Lower Manhattan on a recent afternoon, three sundress-wearers — blocks apart — said a sundress can be any color that makes you happy. Though each acknowledged that being sad in a sundress was also valid.

“You’ve left me no choice but to mansplain women’s fashion,” Randy Trembacki told viewers on TikTok in May. Gesturing around the empty space where he would insert an image of a mini dress from Shein, Mr. Trembecki, a 30-year-old podcast producer based in Texas, named some features of a sundress: fitted top, flowy bottom.

On the phone this month, he elaborated: “It’s conservative but revealing. You know music videos circa early 2010s, where it’s the farmer’s daughter type thing?”

But he acknowledged that his viewpoint was not universal. Much of the feedback he received on his original TikTok came from Black viewers with different ideas about the quintessential sundress.

In “Sundress Pt. 2,” Mr. Trembacki addressed comments like: “Ask any black person what a sundress is and you’re gonna have the OPPOSITE answer.” In response, Mr. Trembacki included a clingy slip by Skims as an example of a sundress.

“The Black community’s preference for form-fitting, long dresses might emphasize a different aspect of allure, one that focuses on visual appeal and the celebration of body contours,” said Shelby Ivey Christie, a fashion historian and former board member of the Black in Fashion Council.

Dictionary definitions of “sundress” typically stipulate sleevelessness.

But how thick is a strap before it becomes a sleeve? Do you have to see shoulder? What about tube tops?

James Hamilton Butler, the director of the associate degree fashion design program at Parsons School of Design, shrugged off the question. Talking about sleeves is outdated, Mr. Butler wrote over email. “We can be who we want without fear of judgment. (Not sure about tube tops though!)”

Sophie Strauss, who calls herself “a stylist for regular people,” says the question of sleeves depends on what the wearer wants to get out of the sundress. In sundress-happy Los Angeles, she sees clients gravitate toward the garment because it tends to “play up parts of women’s bodies we’re told to play up, and downplay parts we’re told to hide,” she said, rattling off brands with big puffy sleeves.

Mr. Trembacki, the TikToker, was not so dogmatic on straps either. “There should be some type of strap,” he said. “Though, there could be no strap, too.”

At some point in recent years, the sundress — traditionally homely and demure — came to take on a peculiar sexual charge. (At least for those who are extremely online.)

On the meme database Know Your Meme, a riff on Maslow’s hierarchy of needs replaces survival requirements like “water,” and “friendship,” with a refrain about sundress-induced activity, too vulgar to print.

What is it that makes “men go crazy for ‘the sundress,’” as a user on X recently put it?

Kyle Brown, a writer who lives in the Williamsburg neighborhood of Brooklyn and has a bicep tattoo of Joan Didion, offered some insight into the contemporary male gaze.

“It’s all about this pastoral American fantasy,” Mr. Brown said, describing a passionate scene involving a man who has come in from doing yardwork to find his sundress-clad wife in the kitchen baking bread. “Men are confused.”

On the street, more practical considerations still prevail.

Lexi Hide, a photographer who was wearing a Chopova Lowena dress on Fifth Avenue on a hot day, explained her reasoning. “I was thinking that a sundress has to be airy enough to make you not want to wear underwear.” She clarified that she just likes how it feels. “Nice warm breeze,” she said.

It may be that the sundress is more of an idea than an article of clothing. After canvassing Lower Manhattan for a potential consensus, I stopped in to Reformation, a clothing store some consider the mother ship of sundresses.

I couldn’t remember the particular sundress Ms. Strauss, the personal stylist, had mentioned, only that it was named after a type of pasta. When I asked a saleswoman for help, she encouraged me to consider any dress in the store. A sundress is whatever you want it to be, she said, pointing me to a mini fit-and-flare in the shade “Last Tango.”