I've seen the term "Programming Platform" coined around. I'm probably able to name a pair of them, but I'm having trouble phrasing what a "Programming Platform" actually mean. Is there any strict definition for it?
There is no strict definition, since a programming platform may mean various things. But it is always about an environment in which software is executed.
In your tags you already named 2 examples: .Net and Jakarta EE. But it could also, for example, be just a browser.
Speaking for Jakarta EE, the environment is the Java VM running on any supported OS and on top an application server which implements the Jakarta EE specification. Your software then, would then run on this platform.