A chiplet is a full-fledged CPU with many cores on it. The term is used when multiple of these chips are stitched together with a high speed interconnect and plugged into the single socket on your motherboard.
If you ripped the lid off a Ryzen "chip", you would see multiple CPU dies underneath for the high end models.
Additionally - MCM - multi-chip module - instead of putting separate chips for various functions on a board, they're fused together in what from the outside looks like a single chip, but internally is 3 or 4 unrelated chips.
If you ripped the lid off a Ryzen "chip", you would see multiple CPU dies underneath for the high end models.