A nub of flesh on the rim of the anus.
It's best when it's placed on top of the anal rim, because then it really resembles a clit, making the asshole look like a tiny cunt.
Some find it pretty cute, calling it mini-cunt or baby cunt. Others find it a bit gross.
The nob is usually pretty sensitive, but not necessarily in a bad way. During anilingus it may even increase the pleasure. If the rimjob is done well, it can even lead to orgasm, hence the second reason why it's called an ass clit.
Having an ass clit myself, I was motivated to read more about them, and I also had to personally go through different stages of dealing with it. First I thought it was gross and never dared asking my wife to give me a rimjob, though only the idea of it was turning me on. I was however giving her rimjobs quite often, so it wasn't long till she insisted on returning the favor. I couldn't belive how good it could feel. Used to think they exagerate in porns, but it was quite the contrary. The area was so sensitive (and particularly the nub), that it took me only a couple of minutes to orgasm. And it was a totally different orgasm.
It doesn't always work, especially if done too often. But we've noticed that if done like once every 2 weeks, the effect is most intense (usually I can feel when I'm 'ready' for it). My wife also loves doing it, as she enjoys my reaction when I orgasm. She says the spasms of my body are similar to a woman's when I orgasm, which turns her on. I off course love it as well, as not only is the anilingus itself very pleasurable, but the orgasm is at least twice as strong as my regular one.
So, if good hygiene happily meets open minds, such a nub is no drama - nothing to be ashamed/frustrated of - but a genuine anal clit! Learn to enjoy it! ;)