1. An exact copy of a product, made legally or illegally, from documentation or by reverse engineering, and usually cheaper.
E.g. "PC clone": a PC-BUS/ISA, EISA, VESA, or PCI compatible x86-based microcomputer (this use is sometimes misspelled "klone" or "PClone"). These invariably have much more bang per buck than the IB PCM they resemble.
E.g. "Unix clone": An operating system designed to deliver a Unix-like environment without Unix licence fees or with additional "mission-critical" features such as support for real-time programming.