Threshold-voltage mismatch
Beginner
Tiny random differences between switches that were meant to be identical. At this size, a few atoms out of place make a difference.
Novice
Random variation in the switching threshold of transistors drawn the same size, mostly from the random number and placement of dopant atoms. Smaller transistors vary more, which hurts circuits like SRAM cells that rely on two halves matching.
Expert
Local (within-die) variation whose standard deviation scales roughly as (Pelgrom). It is why SRAM bitcells, built from the smallest devices on the die and replicated millions of times, must be designed for tails and why their stays high.
Explained in Memory cells (Transistors).
See also: Threshold voltage (Vt), Static noise margin (SNM).