A Rambus in-line memory module (RIMM) packages RDRAM chips onto a printed circuit board for use in high-performance computer systems. Unlike standard SDRAM, these modules run at extremely high clock speeds and require a continuous high-speed bus. This serial-like architecture means any empty motherboard slots must be filled with dummy continuity modules to properly terminate the signal path.
You can't just leave that slot empty, you need to stick a continuity RIMM in there or the bus won't terminate.
RIMMs are notorious for running hot and almost always feature stamped metal heat spreaders covering the chips.
Related terms
DIMM
Dual in-line memory module, a printed circuit board holding memory chips used as main system RAM.
SIMM
A single in-line memory module is an older type of circuit board that holds memory chips.
SODIMM
A compact memory module designed for laptops and space-constrained systems.
Terminator
A terminator is a resistor or circuit placed at the end of a transmission line to absorb signals and prevent reflections.