AI refers to Artificial Intelligence.
In the Series
Several AI's appear throughout the series, mainly as background characters or as holographic adverts. The most notable AI in the series is Poe, the proprietor of The Raven Hotel.
From conversations Poe has with various other AI's, many "sentient" AI's own various businesses, ranging from hotels, to brothels to gambling. According to dialogue, many AI owned business are in decline as people do not like the business behavior of the AI's, which has been likened to "stalkers" and "clingy ex-girlfriends".
Most sentient AI's view humans with disgust. They see them as weak and in need of being concurred. Poe is the exception to this, as he views humans as interesting.