アホ毛, literally, "stupid hair". A single (usually short and fat) lock of hair that sticks out of a character's head. Essentially a much smaller and stylized cowlick. Example characters include Himeko, Saber, Kongou, and Me-tan.
Although the character design element was popular for decades before that, the name ahoge in particular was popularized by Pani Poni Dash!, whose resident hyperactive airhead Himeko's ahoge was named as such directly by the characters. (And was a plot point.)
Hence, while it is called "idiot hair" for that character, and many notable idiot characters have it, it's been in place long enough that a huge number of non-idiot characters have them as well.
To be differentiated from antenna hair, which is two or more thin locks of hair as opposed to only one.
NOT to be confused with ahegao.