System platform - Wikipedia

A system platform is the basic technology of a computer system's hardware and software that defines how a computer is operated and determines what other kinds of software can be used.

Examples:

Wintel, that is, Intel X86 or compatible hardware and Windows operating system.
Macintosh, custom Apple Computer hardware and MacOS operating system.
X86 with UNIX variants such as Linux distributions or BSD variants.
Old home computer microcomputers.
A mainframe with its custom operating system, say an IBM System/360.