The Free On-line Dictionary of Computing (30 December 2018):
The original multitasking operating
system that NeXT, Inc. developed to run on its proprietary
NeXT computers (informally known as "black boxes").
NEXTSTEP includes a specific graphical user interface, an
interface builder, object-oriented application builder, and
several "kits" of prebuilt software objects such as the
Indexing Kit for databases. This software runs on top of
NeXT's version of the Mach operating system on NeXT,
486, Pentium, HP-PA, and Sun SPARC computers.
The official spelling changed from "NeXTstep" to "NeXTStep" to
"NeXTSTEP", and finally "NEXTSTEP".
The last release of NEXTSTEP was 3.3, which NeXT then
developed into "OpenStep".
TjL's Pages (http://peak.org/~luomat/).
See also: GNUStep.