Why are flight instructors in the US called CFIs?
Certified flight instructor = CFI. Add a multiengine rating to your CFI and now you are a multiengine flight instructor, or MEI. An instructor with single and multiengine instructor ratings is commonly referred to as a CFI/MEI.  What if you are a CFI but only in gliders?  CFI-G is the most common shorthand.
Pilot authorizations are specifically called &quot;certifications&quot; in the US, not &quot;licenses.&quot;  My guess at the inclusion for the C in CFI is that any pilot can provide instruction but only appropriately certified and rated instructors can provide training that can be counted for a student's training for a new certification, flight review, or whatever.