Live CD，又译为自生系统，是事先保存于某种可移动保存设备上，可不特定于计算机硬件（non-hardware-specific）而启动的操作系统（通常亦包括一些其他软件），不需安装至计算机的本地外部存储器 - 硬盘。采用的介质包括CD-ROM（Live CD），DVD（Live DVD），闪存盘（Live USB）甚至是软盘等。退出自生系统并重新开机后，电脑就可以恢复到原本的操作系统。自生系统的运作机制，是透过「把原本放在硬盘里的文件，放到内存的虚拟磁盘中」来运作；因此系统内存愈大，则运行速度愈快。

A live CD, live DVD, or live disc is a complete bootable computer installation including operating system which runs in a computer's memory, rather than loading from a hard disk drive; the CD itself is read-only. It allows users to run an operating system for any purpose without installing it or making any changes to the computer's configuration. Live CDs can run on a computer without secondary storage, such as a hard disk drive, or with a corrupted hard disk drive or file system, allowing data recovery. A live ISO is an ISO image of a Live CD which can be used in virtual machine environments, mounted as if it were a CD/DVD and used as the virtual machine's boot CD. Live CDs, ISOs, and images usually include an operating system available without charge or restrictive license such as Linux, rather than a commercial one such as Microsoft Windows, for legal rather than technical reasons.