Half-cylinder plated edge of a board, used so a module can be reflow-soldered onto a carrier (like the Stamp).
How it looks
Drawing: CORE3588 has no castellated pads (its module plugs into connectors instead)
The edge of a small module from above. Each half-circle is a copper-lined hole that was cut through the middle when the board was trimmed.
How it works
Where it fits
Another name for the same idea: castellated / half-hole pads. A cheap way to mount a SoM without a connector. CORE3588 uses board-to-board connectors for its module instead.