Scott Kodai of California State, Chico, contributed this definition on a listserve we’re both on. I thought it was worth putting out here in public space, so, thanks Scott!
“When the Java Virtual Machine (JVM) starts on the server, it allocates some space for Heap Memory. As the system is used, it creates objects in this space and, as those objects are no longer needed, a java garbage collection process reclaims the space. (Note that this is very different than the Oracle Garbage Collection process).
So JVM Heap Free Space means that amount of heap memory which had been allocated and is still available for use. When it gets close to zero, the java garbage collection process will start to impact the server’s performance significantly… When it actually gets to zero (or at least to small enough that new objects can’t be created), you will see “out of memory” errors show up in the logs and the server will, essentially, crash.
You can see this by looking in the Weblogic Console for your server and also in the WebCTServer.JVMMemory.<timestamp>.log files in the WebCTDomain/logs directory (at least if you’re on AP2 or later… previous versions had this information in the WebCTServer.<timestamp>.log).”