A UniFi Access Point (as well as the UniFi controller) in its factory-default state can have two possible default username / password combinations:
| Username: ubnt
After adoption, the device username and password (to be used to SSH for example) can be found under the UniFi Controller Settings > Site > Services section > Device Authentication. This is is also where the administrator can change the password if desired.
In firmware versions prior version 3 (when the multi-sites feature came along), the username and password (post adoption) corresponded to the administrator username / password as was configured in the UniFi controller.