In the world of Japanese animation, fans may describe their favorite characters or archetypes in all sorts of ways, from the main -dere types like tsundere or kuudere to the term "himbo," but no anime character term is more popular -- and potentially divisive -- than the concept of a waifu.
There may be some misconceptions about what a waifu is. This does not describe a cheap fan service character or someone found in a protagonist's harem, but rather the ideal imaginary partner for an anime fan. Thus, choosing a waifu is far more than hosting a simple beauty contest for anime girls; it's a way to find the best hypothetical partner who checks off all the boxes for not just beauty but also quality of character, skills/talents, sense of humor, intelligence and more. A real waifu is the complete package.
Where the Concept of Anime Waifus Came From
The term "waifu" originates from Japan itself, all the way back in the 1980s. According to Dictionary.com, Japanese people of the time sought a more progressive way to describe their spouses, since the Japanese word for wife, kanai, had heavy implications of someone being bound to the house, as in a homemaker. The Japanese therefore borrowed the English word "wife" and pronounced it their own way -- "waifu" -- and used it as a more neutral, general term for female spouses without the homemaker implications of kanai.
Later, in the early 2000s, the loanword "waifu" started being associated with the world of Japanese animation when the word was used in a brief scene in Azumanga Daioh!, a popular slice-of-life comedy anime of the time. A male teacher named Mr. Kimura, when asked, explained that the beautiful woman in a photograph was his wife, but he didn't describe her as his kanai. He said "my wife" in English, perhaps just to sound cool, and thus, the concept of anime waifus was born. In the intervening years, the waifu concept rapidly expanded, taking on a whole new meaning of its own. In fact, it is rarely used to describe an actual female spouse by this point.
How Modern Anime Fans Define Their Waifus
In the modern anime community, the term waifu describes not a married anime character but rather a female anime character who is a fan's ideal romantic partner. It's roughly similar to the concept of having an imaginary friend, except the anime fan can directly lift a character from a work of fiction rather than conceptualize an imaginary friend from scratch. In particular, a waifu is more than the anime fan's favorite female character from a given series; that waifu satisfies all the viewer's needs for an ideal girlfriend in terms of not just her appearance but also her wit and intelligence, personality, skills/talents, compassion and possibly more than a few moe traits. It's a case of "she's totally my type!", which sets waifus apart from the more general concept of a Best Girl. It's possible for one anime fan to view a character as both their waifu and the series' Best Girl, although those two concepts have somewhat different criteria.
By nature, a waifu is purely subjective, and nominating a waifu is typically done simply for fun and novelty. In a way, evaluating female anime characters can be an anime fan's way of figuring out their taste in women, using the convenient context of anime girls to refine their tastes and preferences in a partner. Of course, whether or not the fan uses that information to seek out a real-life partner is an entirely separate matter, and perhaps some fans are content to figure out their ideal waifus and leave it at that.
Meanwhile, the concept of a waifu can both unite and divide anime fans, since fans often take this pretty seriously, similar to how communities may fiercely debate their favorite musical bands and styles despite a lack of objective answers. Some of the most popular waifus may earn some digital high-fives on the Internet among like-minded fans, while more divisive waifus may prompt fans to tease each other with "your waifu is trash" or say something even harsher than that. Some anime fans take the concept quite seriously, while others daydream about waifus just for fun, which can also lead to some awkward conversations on social media and forums. Understandably, plenty of anime fans may elect to keep their waifu a secret, protecting that character from criticism while also having a more pseudo-intimate relationship with them.