A covert channel in which one process signals information to another by modulating its own use of system resources (for example, CPU time) in such a way that this manipulation affects the real response time observed by the second process. - Shon Harris All-in-one CISSP Certification Guide pg 929

Polyinstantiation enables a relation to contain multiple tuples with the same primary keys with each instance distinguished by a security level. When this information is inserted into a database, lower-level subjects need to be restricted from this information. Instead of just restricting access, another set of data is created to fool the lower-level subjects into thinking that the information actually means something else. Source: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002, chapter 11: Application and System Development (page 727).