Debug probe
The adapter between a developer's computer and a microcontroller's debug port — the thing that lets GDB freeze a chip mid-instruction.
USB on one end, JTAG or SWD on the other. The probe translates debugger commands — halt, step, read memory, set a breakpoint — into signals the chip's on-die debug unit understands, and it flashes firmware too. Before probes there were in-circuit emulators, expensive boxes that replaced the entire processor on the board; once the debug logic moved into the silicon, a small adapter was enough.
The familiar names are SEGGER's J-Link, ST's ST-Link, the Black Magic Probe, which runs a GDB server on the probe itself, and Arm's open CMSIS-DAP standard, which turns almost any cheap board into a probe. A common misconception is that the probe decides how many breakpoints you get. It doesn't — the chip's hardware comparators set the limit, often just four to eight, and then you are out.