The amstex package has been renamed amsmath. The old name `amstex' came from the fact that version 1.0 was a more or less straight port into of selected portions from the macro package AmS-/ written by Michael Spivak, but the historical origins of the package are likely of less interest to the general usership—especially looking to the future—than a straightforward declaration in the name `amsmath' of the sort of features that are provided.
Another important reason for the name change was to make possible some progress that would be out of the question (because of the impact on existing documents) if the name `amstex' were retained. For example, unlike amstex, amsmath doesn't automatically load the amsfonts package, as that package was moved out to the AMSFonts distribution as described above. If this change were not accompanied by a name change, many existing documents would have to be updated by hand (adding ||) in order to run without error. A frozen version of amstex.sty is provided for processing pre-existing documents; it won't have active maintainence henceforth except for bug fixes. For new documents authors should use the amsmath package (not just because we advise it, but because it's a better package than its predecessor in a number of ways).