A variety of love song (or sincere lust song) addressed to a specific part of someone's anatomy — their arse.
Generally, this will also involve the singer exhorting the buttocks possessor and the listeners to shake that plump bi-domed anatomical feature, shake it good.
Contrast with Intercourse with You, where the song moves on from specific features to associated activities.
Ross from Friends once got his infant daughter Emma to stop crying by singing "Baby Got Back" to her. Rachel was distinctly unamused, though she ended up resorting to the same song later in the episode.
In an early episode of Mock the Week, John Oliver sang the first few lines of "Baby Got Back" as an example of one of the voices in Tony Blair's head.
Futurama - The Trope Namer, from the episode "A Fishful of Dollars". Fry listens to "Baby Got Back", which Bender later refers as "stuffy old songs about the buttocks".
A Home Movies episode featured a troupe of college-age kids doing a stage workshop on tolerance - a girl sings a song about understanding with lyrics "And you can throw rocks from your house of glass/It ain't no skin off of my ass" - and the song veers off into her referencing her ass over and over.
P. Diddy, Murphy Lee and Nelly - "Shake Your Tailfeather"
Trace Adkins - "Honky Tonk Badonkadonk"
Tyler Dean - "Built for Blue Jeans"
Ludacris - "Money Maker"
Justin Moore's "Back That Thing Up" is a subversion. At first, it sounds like a clone of "Badonkadonk", until he gets to the line "Ain't no time to play today, no rollin' in the hay", which makes it clear that the song really is about her "backing up" the truck.
Destiny's Child made "Bootylicious" a legitimate word in the dictionary.
When Willie Dixon first wrote "Back Door Man" for Howlin' Wolf, the sneaky metaphor was simple enough. By the time The Doors got a crack at the song, the title phrase had acquired additional heft. Don't ask about the lines regarding gustatory preferences.
Before Eddie Murphy was among those actors (Bruce Willis, John Belushi and William Shatner, etc.) who attempted to demonstrate legit singing talent for the pop charts — he probably hopes we've forgotten about that — his first comedy album included the deliberately goofy "Boogie in Your Butt". The lyrics read like a list of emergency-room horror stories.
Sisqo's Thong Song, which also has a bit of foot fetishism towards the end.
"Whoot There It Is" by 95 South (which, incidentally, is older than the more famous Tag Team hit "Whoomp There It Is").