Reserved for exclusive use by something. The term might mean only writable by whatever it is sacred to.
For example, "Register 7 is sacred to the interrupt handler" would mean that if any other code changed the contents of register 7, dire consequences would ensue.
the power, being, or realm understood by religious persons to be at the core of existence and to have a transformative effect on their lives and destinies. Other terms, such as holy, divine, transcendent, ultimate being (or reality), mystery, and perfection (or purity) have been used for this domain. "Sacred" is also an important technical term in the scholarly study and interpretation of religions