What does the "work item" stand for in FPGA?

FPGA hardware is not fixed and the resulting circuit can be vastly different depending on the kernel. Regardless of the kernel programming type (NDRange or single work-item), one or multiple pipelines are generated by Altera's compiler and in each case, either the work-items (for NDRange) or loop iterations (for single work-item) are considered as inputs to the pipeline(s), and are issued in some order and interval decided by the compiler. You can think of single work-item kernels as NDRange kernels that have been wrapped inside a for loop from 0 to global_size; in other words, in single work-item kernels, the loop iterations play a role similar to work-items in NDRange kernels.