An American National Standard for Forth, ANSI X3.215-1994 ("ANS Forth"), is accepted worldwide as the definitive Forth standard.
Prior Forth standards include the Forth-83 Standard and the Forth-79 Standard issued by the Forth Standards Team. The earlier FIG-Forth, while never formally offered as such, was a de facto "standard" for some years. The most recent standard, ANS Forth, defines a set of core words and some optional extensions and takes care to allow great freedom in how these words are implemented. Although there is no need for the individual programmer to comply with the ANS Standard, it does provide a useful means of publishing and porting code between systems. The range of hardware which can support an ANS Forth Standard System is far wider than any previous Forth standard and probably wider than any programming language standard ever.
Copies of the standard cost $193, but the final draft of ANS Forth is free and available (subject to copyright restrictions) via ftp in Postscript and Microsoft Word versions.
The (un)official ANS Forth document is available in various formats including a HTML version for on-line browsing. Be aware, though, that this is a very large document (over 1mb) and it might be better to download the zipped version for off-line browsing.
Two unofficial test suites are available for checking conformance to the ANS Standard Forth: