The term application server is a bit vague - going by Wikipedia
An application server is a software framework that provides an
environment in which applications can run, no matter what the
applications are or what they do.1 It is dedicated to the efficient
execution of procedures (programs, routines, scripts) for supporting
the construction of applications.
It also explain the term for web applications saying
Later, the term took on the meaning of Web applications, but has since
evolved further into that of a comprehensive service layer. An
application server acts as a set of components accessible to the
software developer through an API defined by the platform itself. For
Web applications, these components are usually performed in the same
machine where the Web server is running, and their main job is to
support the construction of dynamic pages. However, present-day
application servers target much more than just Web page generation:
they implement services like clustering, fail-over, and
load-balancing, so developers can focus on implementing the business
logic.[3]
Going by these definitions and comparing with app servers from java world, .NET Framework along with infrastructure support from window server (e.g. IIS/Web Server) will be an application server.