ILP wall
Beginner
The limit on how many steps of one program a core can do at once. Many steps must wait for the step before, so a wider core stops helping.
Novice
Diminishing returns from instruction-level parallelism (ILP), the number of instructions in one program that can run at the same time. Dependences between instructions, branches and cache misses cap it, so making one core wider or deeper gains less and less.
Expert
Even an ideal window is bounded by the critical path of the dataflow graph; real cores also pay for mispredictions and misses. Tullsen et al. measured under 1.5 IPC on an 8-issue model; the cost of wider issue (rename, wakeup, bypass) grows faster than the IPC it buys.
Explained in Multicore and vector units (Architectures).
See also: Multicore processor, Simultaneous multithreading (SMT).