Someone who uploads youtube videos, particularly someone who has some what of a fan base. Although it can mean anyone in youtube, it specifically means someone who makes videos.

when you're hungover as shit after a spending a night drunk as a skunk

A miserable prick who doesn't want to find a real career so they sit in front of a computer ranting for hours until they get enough followers they can manipulate into giving money to pay their mortgage

If you create content on Youtube, watch people who create content on Youtube, if you interact with people on Youtube via comments or other social networks, if you have a connection to people on the website, then you are part of the Youtube community, and therefore, a youtuber.

A person who makes videos on youtube for entertaiment means.

someone who is a member of the youtube community.

BTW: if you know what urbandictionary is and not youtube, than you better ask somebody.

Someone who posts clips or pontificates on YouTube. Usually a brain-dead nonentity with no friends who is idolised by cretinous preteens who have no life.