In computer software, the sandbox is the informal name given to the limited execution environment in which untrusted programs are run. The boundaries of this notional box limit the scope of a malicious program to cause damage to the computer system as a whole. In the Java system, for example, most applets are run in a software sandbox. It is named after the sand-covered area in a children's playground where the children can play with minimal risk of injury from falling.
Similarly, in software development the term sandbox is used to refer to a test system which replicates (often in reduced size or capacity) the actual computing environment for which software is being developed. The presense of such a safe, controlled environment allows developers to try experimental code without fear of damaging a mission-critical system.
In Military training a sandbox is a box of sand used in conjunction with military models to model terrain and demonstrate tactics.