ANS Forth considers a Forth system as consisting of several address spaces, of which only data space is managed and accessible with the memory words. Memory not necessarily in data space includes the stacks, the code (called code space) and the headers (called name space). In Gforth everything is in data space, but the code for the primitives is usually read-only.
Data space is divided into a number of areas: The (data space portion of the) dictionary1, the heap, and a number of system-allocated buffers.
In ANS Forth data space is also divided into contiguous regions. You can only use address arithmetic within a contiguous region, not between them. Usually each allocation gives you one contiguous region, but the dictionary allocation words have additional rules (see Dictionary allocation).
Gforth provides one big address space, and address arithmetic can be performed between any addresses. However, in the dictionary headers or code are interleaved with data, so almost the only contiguous data space regions there are those described by ANS Forth as contiguous; but you can be sure that the dictionary is allocated towards increasing addresses even between contiguous regions. The memory order of allocations in the heap is platform-dependent (and possibly different from one run to the next).
 Sometimes, the term dictionary is used to refer to the search data structure embodied in word lists and headers, because it is used for looking up names, just as you would in a conventional dictionary.