Optical Zoom - definition
The zoom-feature found in many cameras and camera phones that makes subjects appear closer than they actually are (so they fill more of the image area). To create that magnification effect cameras use a set of moving optical lenses - hence the name "optical zoom".
The alternative to optical zoom is digital zoom, which is a highly advertized feature on many consumer devices equipped with a camera, but it doesn't offer many real advantages.
Digital zoom generally influences quite negatively the image quality of the captured images.