The Book ESL Design and Verification: A Prescription for Electronic System Level Methodology (Systems on Silicon) defines ESL as:
the utilization of appropriate abstractions in order to increase comprehension about a system, and to enhance the probability of a successful implementation of functionality in a cost-effective manner.
This combines the idea of system, of abstraction, and (implicitly) the idea of concurrent specification, design of hardware and software, in that it talks about “implementation of functionality” without regard for whether that is to be done in hardware, software or a combination. It also implies a process (where comprehension of the system is increased by using abstractions, and this has a downstream beneficial effect on implementation), some level of system optimization (cost-effectiveness) and proven benefit (appropriate abstractions).
In a more pragmatic manner, ESL is often deemed to be "everything above RTL."