A brainchild is not a child. It is an idea, a plan, a product, a club, a festival, even a whole company — something one person thought up without waiting for a committee, as if the idea were born from their mind. The feeling is “this was theirs; they did not copy it.” We say “the festival was her brainchild” or “the brainchild of Mr Chen.” You will meet this in news, business, and everyday talk: a little colourful, not slang, and not a cold legal word. If many people built the thing together, brainchild is a stretch, unless one person clearly had the first spark. You will almost always meet it as a noun. A few writers use it as a verb, to brainchild something, meaning to think it up — but that sounds odd to many ears, and the noun is the one worth learning. Though actually, once the idea has grown and other people run it, speakers still call it the first thinker’s brainchild, as if the “child” had grown up and walked away.