Through-silicon via (TSV)
Beginner
A tiny metal-filled hole that goes straight through a silicon chip, so chips stacked on top of each other can connect.
Novice
A metal-filled hole that runs straight through a thinned silicon chip, so chips stacked on top of each other, or a chip and the slab of silicon under it, can be wired together vertically.
Expert
Large compared with logic gates and surrounded by keep-out zones where stress would change nearby transistors, so their number and position are early layout decisions. Each TSV process adds steps that can fail, which raises the value of testing each die before it is stacked.
Explained in After tapeout (Extras).
See also: silicon interposer, HBM (High Bandwidth Memory), known-good die.