Stuff left behind, shit that no one wants. From Latin. Often used when the speaker expects that the listener won't know what the word means. This word is commonly used to describe the pile of bones and feathers that the hawk leaves behind after he eats the pigeon.
* Is that your detritus there?
* Why is all this detritus in the back seat of my car?
A useless person, someone that fails to be good at necessarily anything. Trash.
After her career collapsed, Lindsey Lohan became detritus.