XMC module in electronics industry
I found it on Wikipedia: https://en.wikipedia.org/wiki/PCI_Mezzanine_Card


  XMC, or Switched Mezzanine Card, (PMC with high-speed serial fabric interconnect) defined by the VITA 42 standard. XMC specifies a fifth connector ("P15") that supports PCI Express (VITA 42.3) or other high speed serial formats such as Serial RapidIO (VITA 42.2) and Parallel RapidIO (VITA 42.1).


Looks like the acronym used X instead of S, perhaps to avoid confusion with SMC connectors.