Heap - A memory space used for dynamic buffer management and/or dynamic allocation of memory as requested by an application - heap space is memory outside the data, text, and stack segments and is most often reserved by the boot or RTOS during initialization.
High Availability - A system which guarantees that it will be ready to provide services with a quantifiable reliability - e.g. a system is said to provide 5 9ís availability if it is ready to provide service upon request 99.999% of a given year (I.e. is only unavailable for a total of about 5 minutes per year). Note that HA systems can crash, but they canít be out of service very long if they do.
High Reliability - A system which has been designed to have a very low probability of failure to provide services - typically measures such as redundancy, cross strapping, and fail safe modes are designed in to ensure that critical services have an extremely low likelihood of failure.