A bromance is typically defined as a close, platonic relationship between male "heterosexual life partners". Or simply, "bros".
For Danbooru, this is an outward showing of love, affection, and emotion between true bros. The kind that can only emerge between two or more close males who aren't afraid to show their care for each other. Pictures may run the gamut from sadness to laughter to camaraderie and canon may or may not apply.
Pool creator said:
As a work in progress operating under a nebulous definition and concept, there's always room for change. I don't think typical brohavior as applied to females is totally outside the scope of the scope of the pool.
Bromance is definitely NOT to be confused with yaoi and shounen-ai. It is for non-romantic, non-yaoi images only, and anything to the contrary should be expunged. In other words, a hug or manly embrace is fine; blushing and longingly looking into each other's eyes while doing so isn't. There IS a line between the two.
Prime examples of series with heavy bromantic themes include Tengen Toppa Gurren Lagann and some Gundam installments.
Just use your common sense and add at will.
Female Bromance are now included