An imaginary non-existant concept designed to help writers feel less guilty when they lack the mental capacity to write well for a certain time period. The real reason for this temporary lack of creativity is usually one of the following: distraction, health problems, lack of focus, laziness, or sometimes just the basic idea of writer's block can scare a writer into actually getting it.
Friend: Dude, your eyes are red, your hair is messy, the vain in your neck is throbbing and youve got that nervous twitch again!
Writer: I had writer's block! i was up all night trying to finish the chapter/script/copy but im just not able to do it! Whats going on with me??!!!!!