Definition of lock

1. (on the Macintosh) to mark a file or disk as "Do not change" by clicking on the "Locked" box in the "Get Info" window.
2. (in various operating systems) to mark a file as in use so that other programs running concurrently will not change it.
3. to prevent a diskette from being written on.A 3½-inch diskette can be physically locked by sliding the small plastic tab (on the back of the diskette) toward the edge. This uncovers a square hole that tells the disk drive that the diskette is " read-only." This is called write protection. (See write-protect.)

Any diskette sent to a service bureau should be locked so that viruses cannot be written on it.