The term Heapdump describes the IBM® Virtual Machine for Java™ mechanism that generates a dump of all the live objects that are on the Java heap; that is, those that are being used by the running Java application.
This dump is stored in a Portable Heap Dump (PHD) file, a compressed binary format. You can use various tools on the Heapdump output to analyze the composition of the objects on the heap and (for example) help to find the objects that are controlling large amounts of memory on the Java heap and the reason why the Garbage Collector cannot collect them.