While defining our application architecture (which contains both web UI and external web services) we stumbled upon our inability to find a common name for the topmost layer. It is quite clear that there are DAL (data access layer) and BLL (business logic layer) in our application. On the top of BLL, there reside the UI, which is commonly called the presentation layer (e.g., http://msdn.microsoft.com/en-us/library/ff647339.aspx). But there's also a service layer, which resides on the top of BLL as well!
But a cake can't have two layers on the same level :) So please help me to find a term for that.
The term API (Application Programming Interface) is commonly used for programming interfaces, though in this case "Service Layer" may be more appropriate and descriptive.
Layers can be side by side - nothing says they have to be one on top of another.