OpenStep是一组面向对象的API，大部分是由NeXT与Sun Microsystems开发。作为一种面向对象的操作系统的规范，有一些现代操作系统用它作为核心。对于OpenStep一个重要的认识是，它只是一组API规范，而OPENSTEP（一定要大写）是由NeXT开发的一个OpenStep的特殊实现。尽管它构建于Mach－即基于Unix（也是NEXTSTEP的核心），OPENSTEP的同样有在Solaris和Windows NT上的版本。此外，OPENSTEP库（由OPENSTEP操作系统提供的库）实际上已经成为了原始OpenStep规范的超集。

OpenStep is an object-oriented application programming interface (API) specification for a legacy object-oriented operating system, with the basic goal of offering a NeXTSTEP-like environment on a non-NeXTSTEP operating system. OpenStep was principally developed by NeXT with Sun Microsystems, to allow NeXTSTEP (like) development on Sun's operating systems, specifically Solaris. NeXT produced a version of OpenStep for their own Mach-based Unix, known as OPENSTEP (all capitalized), as well as a version that ran on Windows NT. The software libraries that shipped with OPENSTEP are a superset of the original OpenStep specification, including many features from the original NeXTSTEP.