what is a "Platform"?

Usually the answer to the question “What is X”, at least in the context of software engineering, is given in two different ways:
- What does X do or what is X supposed to d.
- How does X work.
The former is more of philosophical answer and the latter more of a pragmatic one. For example, “what is a service?” could be answered in one the two ways:

-  A unit of functionality that is exposed thru a well defined interface and is loosely coupled to its consumers, it is autonomous, reusable, discoverable and stateless.

Or it can be answered as

-    It is a unit of code exposed thru a WSDL and invoked using SOAP and it is language neutral

Those who know me know that I am more inclined toward philosophy. So when I attempt to answer “what is a platform?” – as I had to recently when we were building eBay Application Platform - I opt for what it does.
To me the answer is simple, at least in the realm of software engineering:
A software platform is any set of functionality that increases developers’ productivity, plain and simple.
Operating systems do that, languages do that, APIs do that so do IDEs such as Eclipse. So what is the difference between tools and platforms? Tools are not programmable, platforms are. In other words developers can “program” platforms to suit their needs. In other words tools are used to accomplish one task, platforms can be used (i.e. programmed) to perform different tasks. Some platforms start as tools (like Eclipse, Excel) but evolve to become a platform.

Why, besides philosophical clarity, is this important? It can be used to define a clear goal and metrics for success of whatever is called a “Platform”.