"Alexa 18" could refer to a specific feature or capability that was updated or fixed. Without more context, it's hard to say what "18" specifically refers to. It could be the 18th feature added to Alexa, an update numbered 18, or something similar.
: One major fix is Alexa’s new ability to process nuance. Instead of needing exact device names, you can now say, “Alexa, it’s too bright in here,” and the AI will infer that you want to dim the lights. alexa 18 fixed