ISA Legacy Interrupt - Industry Standard Architecture Legacy Interrupt, specifically refers to x86 architecture IRQ0-15 which have been part of the x86 architecture from the beginning (8086) and support a number of well-known PC devices and services such as booting from a hard drive.
Isochronal - literally the same in time, which in real-time systems means that a service is required to produce a response at a precise time relative to a service request - not too early and not too late. This is important to continuous media applications and digital control which are sensitive to jitter. Most often isochronal services hold a response computed ahead of deadline that is delivered to an interface within a narrow band around the optimal time.
ISR - Interrupt Service Routine, the application level of an interrupt handler which is often a call-back function registered with an RTOS that installs the interrupt handler at an interrupt vector.